Delete ApiBp2build soong_build mode

This feature is obsolete.

This makes a large number of codepaths "dead code" (such as
module-specific implementations of ApiBp2build functionality). These
will be deleted in a followup CL.

Bug: 284029211
Test: Presubmits
Change-Id: Ib53b99f1fe8c24380d219caf44e9bb3b96724fa0