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: Iff95b80171569f7e0842cbcf091f0a18a9ec88f7