Remove getFieldSymbolToBeInitialized

[RikkiGibson]

Closes #75245

It looks like this function is no longer needed because we recently added a mechanism to share a slot between property and backing field under appropriate conditions. Specifically, changes to NullableWalker.GetOrCreateSlot in #75246.

[RikkiGibson]

Looks like there are some cases where preconditions are not properly set in absence of this call. Likely we need to add some similar tests with properties using field keyword and make sure behavior is consistent.

[xUnit.net 00:00:07.68]     Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.RequiredMembersTests.RequiredMemberSuppressesNullabilityWarnings_MemberNotNull_ChainedConstructor_03 [FAIL]
  Failed Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.RequiredMembersTests.RequiredMemberSuppressesNullabilityWarnings_MemberNotNull_ChainedConstructor_03 [24 ms]
  Error Message:
   
Expected:
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property1").WithLocation(11, 9),
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property2").WithLocation(12, 9)
Actual:
                // 0.cs(11,9): warning CS8602: Dereference of a possibly null reference.
                //         Property1.ToString();
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property1").WithLocation(11, 9)
Diff:
-->                 Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property2").WithLocation(12, 9)
  Stack Trace:
     at Microsoft.CodeAnalysis.DiagnosticExtensions.Verify(IEnumerable`1 actual, DiagnosticDescription[] expected, Boolean errorCodeOnly) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 99
   at Microsoft.CodeAnalysis.DiagnosticExtensions.Verify(IEnumerable`1 actual, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 48
   at Microsoft.CodeAnalysis.DiagnosticExtensions.Verify(ImmutableArray`1 actual, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 63
   at Microsoft.CodeAnalysis.DiagnosticExtensions.VerifyDiagnostics[TCompilation](TCompilation c, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 109
   at Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.RequiredMembersTests.RequiredMemberSuppressesNullabilityWarnings_MemberNotNull_ChainedConstructor_03() in /_/src/Compilers/CSharp/Test/Symbol/Symbols/RequiredMembersTests.cs:line 4542
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
[xUnit.net 00:00:08.51]     Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.RequiredMembersTests.RequiredMemberSuppressesNullabilityWarnings_MemberNotNull_ChainedConstructor_04 [FAIL]
  Failed Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.RequiredMembersTests.RequiredMemberSuppressesNullabilityWarnings_MemberNotNull_ChainedConstructor_04 [18 ms]
  Error Message:
   
Expected:
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property1").WithLocation(12, 9),
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property2").WithLocation(13, 9)
Actual:
                // 0.cs(12,9): warning CS8602: Dereference of a possibly null reference.
                //         Property1.ToString();
                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property1").WithLocation(12, 9)
Diff:
-->                 Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Property2").WithLocation(13, 9)
  Stack Trace:
     at Microsoft.CodeAnalysis.DiagnosticExtensions.Verify(IEnumerable`1 actual, DiagnosticDescription[] expected, Boolean errorCodeOnly) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 99
   at Microsoft.CodeAnalysis.DiagnosticExtensions.Verify(IEnumerable`1 actual, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 48
   at Microsoft.CodeAnalysis.DiagnosticExtensions.Verify(ImmutableArray`1 actual, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 63
   at Microsoft.CodeAnalysis.DiagnosticExtensions.VerifyDiagnostics[TCompilation](TCompilation c, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 109
   at Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols.RequiredMembersTests.RequiredMemberSuppressesNullabilityWarnings_MemberNotNull_ChainedConstructor_04() in /_/src/Compilers/CSharp/Test/Symbol/Symbols/RequiredMembersTests.cs:line 4575
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)