[NFC][ASTImporter] remove the unnecessary condition checks in ASTImporter.cpp

I think that these conditions are unnecessary because in VisitClassTemplateDecl we import the definition via the templated CXXRecordDecl and in VisitVarTemplateDecl via the templated VarDecl. These are named ToTemplted and DTemplated respectively.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D128608