Remove SystemUI Compose build flag (2/4)

This CL removes the build configuration around the SystemUI Compose
build flag as well as all files that were used when the flag was
disabled, now that this flag is enabled by default on all SystemUI
variants.

I will remove the ComposeFacade in a follow-up CL.

Bug: 295304993
Test: mp droid
Flag: N/A
Change-Id: I6294e43c411aeaafb77a59b327ce9c4ead07fc95