[clang-tidy] remove private decltypeType in TrailingReturnType

Summary:
This patch removes a private matcher in fuchsia/TrailingReturnType check because
the matcher is now in ASTMatchers

Reviewers: aaron.ballman, alexfh, hokein

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D49618

llvm-svn: 337707