Concurrency issue with fixed fields (#56581)

Author: jcouv

src/Compilers/CSharp/Portable/Emitter/Model/PEModuleBuilder.cs

@@ -1532,17 +1532,6 @@ public NamedTypeSymbol SetFixedImplementationType(SourceMemberFieldSymbol field)
             }
         }
 
-        internal NamedTypeSymbol GetFixedImplementationType(FieldSymbol field)
-        {
-            // Note that this method is called only after ALL fixed buffer types have been placed in the map.
-            // At that point the map is all filled in and will not change further.  Therefore it is safe to
-            // pull values from the map without locking.
-            NamedTypeSymbol result;
-            var found = _fixedImplementationTypes.TryGetValue(field, out result);
-            Debug.Assert(found);
-            return result;
-        }
-
         protected override Cci.IMethodDefinition CreatePrivateImplementationDetailsStaticConstructor(PrivateImplementationDetails details, SyntaxNode syntaxOpt, DiagnosticBag diagnostics)
         {
             return new SynthesizedPrivateImplementationDetailsStaticConstructor(SourceModule, details, GetUntranslatedSpecialType(SpecialType.System_Void, syntaxOpt, diagnostics)).GetCciAdapter();