[cxx-interop] Remove symbolic import mode

Importing C++ class templates in symbolic mode has proven to be problematic in interaction with other compiler features, and it isn't used widely. This change removes the feature.

rdar://150528798

lib/ClangImporter/ImportName.cpp

@@ -2274,11 +2274,6 @@ ImportedName NameImporter::importNameImpl(const clang::NamedDecl *D,
 
   if (auto classTemplateSpecDecl =
           dyn_cast<clang::ClassTemplateSpecializationDecl>(D)) {
-    /// Symbolic specializations get imported as the symbolic class template
-    /// type.
-    if (importSymbolicCXXDecls)
-      return importNameImpl(classTemplateSpecDecl->getSpecializedTemplate(),
-                            version, givenName);
     if (!isa<clang::ClassTemplatePartialSpecializationDecl>(D)) {
       auto name = printClassTemplateSpecializationName(classTemplateSpecDecl,
                                                        swiftCtx, this, version);