Clean up aconfig flag hide_preinstalled_carrier_app_at_most_once

Flag hide_preinstalled_carrier_app_at_most_once has been advanced to
next for a while and there is no regression observed. It's time to clean
it up to keep code healthy.

Bug: 158028151
Test: atest CarrierAppUtilsTest
Flag: EXEMPT flag clean up only
Change-Id: I28d55816eb263ca17cdbb00b5d7a13a42400d4ae