Drop use_source_config_var from prebuilt selection

This CL makes this property a no-op from the perspective of prebuilt
selection. The primary user of this was mainline modules, but they have
now switched to RELEASE_APEX_CONTRIBUTIONS_* for prebuilt selection,
both for internal `main` development and in partner drops.

Test: in internal, lunch cf_x86_64_phone-next-userdebug; m nothing; and
verified there was a path to the prebuilt bt apex

Test: in partner workspace, lunch gsi_partner_arm-ap31-userdebug; m
nothing; and verified that there was a path to the prebuilt adservices apex

Bug: 308188211
Change-Id: I124a6dada5c20aca4bdebf70dcbe5bbec28cefbe