Remove flag_check.py, we now have flag directive check in gerrit presubmit.

See b/340896878  & go/flag-format-checker-design-doc & go/flag-directive-checker

Bug: 391534772
Test: Manual
Flag: EXEMPT remove flag check hook
Change-Id: I1291678e29a96bae65b63db5d6750e1b48c4601f