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This feature is obsolete.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

bp2build/conversion.go

@@ -145,7 +145,7 @@ func createBuildFiles(buildToTargets map[string]BazelTargets, mode CodegenMode)
 		targets.sort()
 
 		var content string
-		if mode == Bp2Build || mode == ApiBp2build {
+		if mode == Bp2Build {
 			content = `# READ THIS FIRST:
 # This file was automatically generated by bp2build for the Bazel migration project.
 # Feel free to edit or test it, but do *not* check it into your version control system.