Drop PRODUCT_PACKAGES special-case in source vs prebuilt mainline selection

google products that include mainline apexes use
RELEASE_APEX_CONTRIBUTIONS_* for source vs prebuilt selection.
RELEASE_APEX_CONTRIBUTIONS is aware of aosp products via a parallel
special-case in core/product_config.mk, so we do not need this duplicate
special case here.

Bug: 308188056
Test: presubmits
Change-Id: I2fc904d3f661c105ec9a48a313723bc881a623f8