Delete build-aml-prebuilts.sh

Its usecase can now be supported natively by soong_ui via the
--skip-kati flag + the mainline_sdk product. Migrate
build-mainline-modules.sh to use this new approach, and delete the
script.

Bug: 174315599
Test: build/soong/scripts/build-mainline-modules.sh
Change-Id: I0f6b34310df51ddf060b2912d43487f4d37dd01b