selftests/resctrl: Remove duplicate codes that clear each test result file

Before exiting each test function(run_cmt/cat/mbm/mba_test()),
test results("ok","not ok") are printed by ksft_test_result() and then
temporary result files are cleaned by function
cmt/cat/mbm/mba_test_cleanup().
However, before running ksft_test_result(),
function cmt/cat/mbm/mba_test_cleanup()
has been run in each test function as follows:
  cmt_resctrl_val()
  cat_perf_miss_val()
  mba_schemata_change()
  mbm_bw_change()

Remove duplicate codes that clear each test result file,
while ensuring cleanup properly even when errors occur in each test.

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>