Retire mainline.mk

We've converted all known users of mainline.mk to use
mainline_system.mk. Don't include mainline.mk directly in the future,
instead include mainline_system.mk and other partition specific
makefiles. Remove mainline.mk to prevent people from accidentally
including it again.

Bug: 141443017
Test: TH
Change-Id: Id161f29951982a100482a6201130adeebdb49846