[arcanist] rm -f for write protected temp files

[nickdesaulniers]

Summary

When running arc diff on a newly added file,
utils/arcanist/clang-format.sh is run. For new files there is no change,
but the script will bail. Without the use of the -f flag, we get
interactive prompts like:
rm: remove write-protected regular file '/tmp/tmp.ReMybrBw35'?

utils/arcanist/clang-format.sh

 readonly src_file
 readonly original_file
 readonly formatted_file
 cp -p "${src_file}" "${original_file}"
 cp -p "${src_file}" "${formatted_file}"
 
 cleanup() {
   rc=$?
-  rm "${formatted_file}" "${original_file}"
+  rm -f "${formatted_file}" "${original_file}"
   exit ${rc}
 }
 trap 'cleanup' INT HUP QUIT TERM EXIT
 
 # Arcanist can filter out lint messages for unchanged lines, but for that, we
 # need to generate line by line lint messages. Instead, we generate one lint
 # message on line 1, char 1 with file content edited using clang-format-diff.py
 #