Address PR feedback and remove unused resource and comment out unused error code

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -4516,15 +4516,6 @@ internal class CSharpResources {
             }
         }
         
-        /// <summary>
-        ///   Looks up a localized string similar to Feature &apos;{0}&apos; is experimental and unsupported; use &apos;/features:{1}&apos; to enable..
-        /// </summary>
-        internal static string ERR_FeatureIsExperimental {
-            get {
-                return ResourceManager.GetString("ERR_FeatureIsExperimental", resourceCulture);
-            }
-        }
-        
         /// <summary>
         ///   Looks up a localized string similar to Feature &apos;{0}&apos; is not implemented in this compiler..
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -4401,9 +4401,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_FeatureNotAvailableInVersion7" xml:space="preserve">
     <value>Feature '{0}' is not available in C# 7.0. Please use language version {1} or greater.</value>
   </data>
-  <data name="ERR_FeatureIsExperimental" xml:space="preserve">
-    <value>Feature '{0}' is experimental and unsupported; use '/features:{1}' to enable.</value>
-  </data>
   <data name="ERR_FeatureIsUnimplemented" xml:space="preserve">
     <value>Feature '{0}' is not implemented in this compiler.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1294,7 +1294,7 @@ internal enum ErrorCode
         ERR_EncodinglessSyntaxTree = 8055,
         // ERR_AccessorListAndExpressionBody = 8056, Deprecated in favor of ERR_BlockBodyAndExpressionBody
         ERR_BlockBodyAndExpressionBody = 8057,
-        ERR_FeatureIsExperimental = 8058,
+        //ERR_FeatureIsExperimental = 8058, No experimental feature
         ERR_FeatureNotAvailableInVersion6 = 8059,
         // available 8062-8069
         ERR_SwitchFallOut = 8070,