Update .github/workflows/format.yml

It seems that the temporary file is unnecessary.
README (https://github.com/google/yapf/blob/master/README.rst) says:

> If --diff is supplied, YAPF returns zero when no changes were necessary, non-zero otherwise (including program error). You can use this in a CI workflow to test that code has been YAPF-formatted.<Paste>

Author: kmyk

.github/workflows/format.yml

@@ -20,5 +20,5 @@ jobs:
         oj-verify -h
         pylint --disable=all --enable=unused-import onlinejudge_verify onlinejudge_bundle tests setup.py
         isort --check-only --diff onlinejudge_verify onlinejudge_bundle tests setup.py
-        yapf --diff --recursive onlinejudge_verify onlinejudge_bundle tests setup.py | tee yapf.patch && test ! -s yapf.patch
+        yapf --diff --recursive onlinejudge_verify onlinejudge_bundle tests setup.py
         mypy onlinejudge_verify onlinejudge_bundle setup.py