Revert "gs101: re-enable prebuilt boot.img/boot-user.img usage"

This reverts commit 7badc4b5f1c7b6adff8cd9142f5c97bd4a5a0d49.

Reason for revert: it introduce b/201469630 and b/201476061

Bug: 201469630
Bug: 201476061
Change-Id: Id48ea931410a55bfd382feb7a0da02994fdf8928
1 file changed