Don't track uninitialized non-null struct fields (#34292)

It' s already an error to fail to initialize a field of a struct, so
this warning is just a duplication of a more serious error condition

Fixes #30022

src/Compilers/CSharp/Portable/FlowAnalysis/UnassignedFieldsWalker.cs

@@ -33,7 +33,7 @@ internal static void Analyze(CSharpCompilation compilation, MethodSymbol method,
                 return;
             }
 
-            if (HasThisConstructorInitializer(method))
+            if (HasThisConstructorInitializer(method) || method.ContainingType.IsValueType)
             {
                 return;
             }

src/Compilers/CSharp/Test/Semantic/Semantics/UninitializedNonNullableFieldTests.cs

@@ -691,13 +691,7 @@ internal S(string s)
                 Diagnostic(ErrorCode.ERR_UnassignedThisAutoProperty, "S").WithArguments("S.P").WithLocation(6, 14),
                 // (6,14): error CS0171: Field 'S.F' must be fully assigned before control is returned to the caller
                 //     internal S(string s)
-                Diagnostic(ErrorCode.ERR_UnassignedThis, "S").WithArguments("S.F").WithLocation(6, 14),
-                // (6,14): warning CS8618: Non-nullable property 'P' is uninitialized.
-                //     internal S(string s)
-                Diagnostic(ErrorCode.WRN_UninitializedNonNullableField, "S").WithArguments("property", "P").WithLocation(6, 14),
-                // (6,14): warning CS8618: Non-nullable field 'F' is uninitialized.
-                //     internal S(string s)
-                Diagnostic(ErrorCode.WRN_UninitializedNonNullableField, "S").WithArguments("field", "F").WithLocation(6, 14));
+                Diagnostic(ErrorCode.ERR_UnassignedThis, "S").WithArguments("S.F").WithLocation(6, 14));
         }
 
         [Fact]
@@ -714,8 +708,7 @@ public void EmptyStruct()
             comp.VerifyDiagnostics();
         }
 
-        // https://github.com/dotnet/roslyn/issues/30022: Struct assign `this`.
-        [Fact(Skip = "https://github.com/dotnet/roslyn/issues/30022")]
+        [Fact]
         [WorkItem(30022, "https://github.com/dotnet/roslyn/issues/30022")]
         public void StructAssignThis()
         {