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:367a4c3d122bb601ac80f64346f7528a4f348812)
Merged-In: I8d02430b70c1ed432826e02bc5938c6772ff1f95
Change-Id: I8d02430b70c1ed432826e02bc5938c6772ff1f95