selftests: amd-pstate: Don't delete source files via Makefile

Revert the portion of a recent Makefile change that incorrectly
deletes source files when doing "make clean".

Fixes: ba2d788a

 ("selftests: amd-pstate: Trigger tbench benchmark and test cpus")
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>