Remove isAppStartModeDisabled binder entry point

Obsolete: the use cases for which it was introduced now have a clean
internal API layer to use instead, and this reduces informational
exposure about package presence & states.

Bug: 185513355
Test: atest SyncManagerTest
Test: atest CtsSyncManagerTest
Change-Id: I4aa1848950061dce3a1378f7d0117364e62ba59f