[NFC][PowerPC] Cleaning up test file and removing redundant front-end test (llvm#151971)

NFC patch to clean up extra lines of code in the file
`llvm/test/CodeGen/PowerPC/check-zero-vector.ll` as the current one has
loop unrolled.
Also removing the file `clang/test/CodeGen/PowerPC/check-zero-vector.c`
as the patch affects only the backend.

Co-authored-by: himadhith <[email protected]>

Author: Himadhith