Clean up flag fix_crash_on_getting_config_when_phone_is_gone

Flag fix_crash_on_getting_config_when_phone_is_gone has been advanced to
next for a while and no regression is observed. Clean it up for now to
keep code healthy.

Bug: 319791612
Test: atest TelephonySubscriptionTrackerTest
Flag: EXEMPT flag clean up only
Change-Id: I6f5ba1998824c87ea458bf8163fdf9e6ba8a451d