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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1ad73f715bfc96abbbee7c21a1026a0054c6c9ae)
Merged-In: I6f5ba1998824c87ea458bf8163fdf9e6ba8a451d
Change-Id: I6f5ba1998824c87ea458bf8163fdf9e6ba8a451d