[clang-tidy] Remove target specification hicpp-signed-bitwise

This patch removes the targetspecification of a testcase, that broke
for ARM. The underlying problem was fixed which makes it unnecessary to
specify the target architecture (problem was the signedness of `char`).

Committing without review was accepted in https://reviews.llvm.org/D38399
by aaron.ballman.

llvm-svn: 314811