Remove framework-atb-backward-compatibility

The class in this library no longer needs to be conditionally included
on the bootclasspath based on the REMOVE_ATB_FROM_BCP flag so is being
built as part of framework-minus-apex.

Bug: 184331423
Test: m nothing
Change-Id: If7c2ec51be0670ffda22e18ce1e028783a1513e7