Drop special-casing of atv in source/prebuilt selection

All tv devices on internal CI branches currently build in trunk_staging
config. Since trunk_staging provides sources of mainline modules via
RELEASE_APEX_CONTRIBUTIONS_*, we do not need this special-casing
anymore.

(cs query
https://source.corp.google.com/search?q=ATV.*TARGET_RELEASE.*next%20f:%5C.gcl&sq=)

Test: presbumits
Test: used go/abtd to test a handful of targets in tv's release branch
Bug: 308188056
Change-Id: Iaadd5b9151c8cce3fe0a193f2e5c37d63e6095d4