Remove NonNullTypesOn/Off helper methods (#36105)

src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests_Nullable.cs

@@ -305,7 +305,7 @@ public void EmitAttribute_LocalFunctionConstraints()
             var source = @"
 class C
 {
-" + NonNullTypesOn() + @"
+#nullable enable
     void M1()
     {
         local(new C());
@@ -329,7 +329,7 @@ public void EmitAttribute_LocalFunctionConstraints_Nested()
 interface I<T> { }
 class C
 {
-" + NonNullTypesOn() + @"
+#nullable enable
     void M1()
     {
         void local<T>(T t) where T : I<C?>
@@ -351,7 +351,7 @@ public void EmitAttribute_LocalFunctionConstraints_NoAnnotation()
             var source = @"
 class C
 {
-" + NonNullTypesOn() + @"
+#nullable enable
     void M1()
     {
         local(new C());
@@ -474,9 +474,9 @@ public void EmitAttribute_Interface_01()
 public class A : I<object>
 {
 }
-" + NonNullTypesOff() + @"
+#nullable disable
 public class AOblivious : I<object> { }
-" + NonNullTypesOn() + @"
+#nullable enable
 public class B : I<object?>
 {
 }
@@ -496,9 +496,9 @@ public class B : I<object?>
 @"class C
 {
     static void F(I<object> x, I<object?> y) { }
-" + NonNullTypesOff() + @"
+#nullable disable
     static void FOblivious(I<object> x) { }
-" + NonNullTypesOn() + @"
+#nullable enable
     static void G(A x, B y, AOblivious z)
     {
         F(x, x);

src/Compilers/Test/Utilities/CSharp/CSharpTestBase.cs

@@ -463,16 +463,6 @@ protected static CSharpCompilationOptions WithNonNullTypes(CSharpCompilationOpti
             return (options ?? TestOptions.ReleaseDll).WithNullableContextOptions(nullableContextOptions);
         }
 
-        protected static string NonNullTypesOff()
-        {
-            return "#nullable disable";
-        }
-
-        internal static string NonNullTypesOn()
-        {
-            return "#nullable enable";
-        }
-
         internal CompilationVerifier CompileAndVerifyWithMscorlib40(
             CSharpTestSource source,
             IEnumerable<MetadataReference> references = null,

src/EditorFeatures/CSharpTest/Diagnostics/Nullable/CSharpDeclareAsNullableCodeFixTests.cs

@@ -20,7 +20,7 @@ internal override (DiagnosticAnalyzer, CodeFixProvider) CreateDiagnosticProvider
         private static readonly TestParameters s_nullableFeature = new TestParameters(parseOptions: new CSharpParseOptions(LanguageVersion.CSharp8));
 
         private readonly string NonNullTypes = @"
-" + Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.NonNullTypesOn() + @"
+#nullable enable
 ";
 
         [Fact]