[clang-tidy] Remove opencl-c.h inclusion from tests

After D120254 some clang-tidy tests started failing on release builds.

clang-tidy has been using the `-fdeclare-opencl-builtins` functionality
since this became the default in clang, so there is no need to include
`opencl-c.h`.

Differential Revision: https://reviews.llvm.org/D120470