Cleanup unused code for bp2build conversion.

This code to support bp2build-specific modules is no longer necessary as
we do not create bp2build modules after https://r.android.com/1792714

Test: build/bazel/ci/bp2build.sh
Change-Id: I20d3c42f7e3516554cfb537fef05ba40d222dbeb

bp2build/testing.go

@@ -225,11 +225,6 @@ type customBazelModuleAttributes struct {
 	Arch_paths       bazel.LabelListAttribute
 }
 
-type customBazelModule struct {
-	android.BazelTargetModuleBase
-	customBazelModuleAttributes
-}
-
 func customBp2BuildMutator(ctx android.TopDownMutatorContext) {
 	if m, ok := ctx.Module().(*customModule); ok {
 		if !m.ConvertWithBp2build(ctx) {