[legacy sf flag] - Remove enableNewFrontEnd()

There is no need to specially enable the new SF frontend
once the legacy frontend is removed. This method can be
removed from any test suites.

Bug: b/330785038
Test: presubmit
Test: atest LayerHistoryIntegrationTest
Change-Id: I2e94156c49b79bbcc779a65997e184a2e4eb1660