Sema: Remove dead code in CheckTemplateArgumentAddressOfObjectOrFunction

Summary:
DeclRefExpr::getDecl gives us back a ValueDecl, this isa<> check will
never fire.

Reviewers: eli.friedman, doug.gregor, majnemer

Reviewed By: majnemer

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1434