Remove unused system_modules

With
https://android-review.googlesource.com/c/platform/build/soong/+/1145829,
android_*stubs_current_system_modules are no longer used. Thus removing.

Bug: 142896162
Test: m checkbuild
Change-Id: I203766d92a8ebac862a816e9b6b68b6d47d5312c