Cleanup : remove change_method_of_obtaining_ims_registration_radio_tech flag

Bug: 409607871
Bug: 417818125
Test: atest PhoneSwitcherTest
Test: manual test and verify based on text log
      after IMS registered -> wifi on -> APM on -> APM off -> wifi off
Flag: EXEMPT (remove flag)
Change-Id: I0248a5a9a816a21949648c4c071359357150b2e3