Remove missed usage of WRN_CallNotInterceptable

Author: RikkiGibson

src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs

@@ -2940,7 +2940,6 @@ public void TestIsBuildOnlyDiagnostic()
                     case ErrorCode.ERR_EncUpdateFailedDelegateTypeChanged:
                     case ErrorCode.ERR_CannotBeConvertedToUtf8:
                     case ErrorCode.ERR_FileTypeNonUniquePath:
-                    case ErrorCode.WRN_CallNotInterceptable:
                     case ErrorCode.ERR_InterceptorSignatureMismatch:
                     case ErrorCode.ERR_InterceptorMustHaveMatchingThisParameter:
                     case ErrorCode.ERR_InterceptorMustNotHaveThisParameter: