Revert "Normalize building generic system image with treble-common.mk"

Revert by Build Cop. This change is causing build failures across builders and branches, e.g.:

ninja: error: 'out/target/product/generic_x86/obj/lib/android.hidl.base@1.0.so', needed by 'checkbuild', missing and no known rule to make it
09:08:08 ninja failed with: exit status 1

This reverts commit 76ada938.

Change-Id: I5e7bfa65a70cf5d61fb233cdbdfe7f420fd5ab51