SF: Remove flag check for modesetting lock

Ie15de10d16eefeb65289574b120d2ef4cc09d6c0, which restored mStateLock for
modesetting to fix races that led to crashes and broken scheduling, was
flagged to the rolled-out connected_display flag, such that the feature
could be rolled back if needed. Now that the fix has soaked, remove the
race-prone code path that is no longer taken.

Bug: 255635711
Flag: EXEMPT removing connected_display
Test: Build
Change-Id: Ib535e92906fb2d8be2bc70b9af29805a00d40e90