Remove "Backup Calling" button from Settings UI

Pixel will not use 'Backup Calling' setting to enable cross-SIM calling.
Code may live on in aosp for other OEMs.

Revert of 9adfa375 with changes to
remove lines with "ToDo Deprecate".

Bug:21153988
Test: Live test for cross-SIM calling across both subs.

Change-Id: I90e04ebf9f7bc3a05f459f59775ea7baa18716da