Remove unused flag - ap_domain_selection_enabled

This change is to remove the unused flag after rolling back a Gantry
workflow.
 - com.android.internal.telephony.flags.ap_domain_selection_enabled

Bug: 258112541
Bug: 433527538
Flag: EXEMPT removal of abandoned flag
Test: TH
Change-Id: I3d9a9d6f586ab7b86814358872d3777bebc4f24a