Rollback computing the ABI in JavacInMemoryStep via an annotation processor.

Summary:
This rolls back a portion of bad922a388035369ab7bfca1d02740711f2049c8 because
apparently JavaCompiler.CompilationTask.setProcessor() overrides any
existing processors that were set via the options list. We do not want
to disable the list of processors set by the user.

Test Plan: Sandcastle builds.