[clang] Remove redundant check-prefix=CHECK from tests. NFC.

Reviewers: t.p.northover, mstorsjo, rsmith, mcrosier

Reviewed By: mstorsjo, mcrosier

Subscribers: mcrosier, javed.absar, cfe-commits

Differential Revision: https://reviews.llvm.org/D35465

llvm-svn: 308192