Remove obsolete is/setPermissionEnforced().

isPermissionEnforced() always returns true and is only exposed on AIDL
and there is actually no API, so it is directly removed.
setPermissionEnforced() currently doesn't affect anything so its
remaining implementation is removed as well, and we consider all
permissions enforced when needed for compatibility of dumping.

Bug: 158736025
Test: presubmit
Change-Id: I0584553ac0171147b6f131b5359ddb2964113a1d