Remove per-file CompOS signature support

Delete the now unused proto and all related code.

I marked a bunch of functions static (or in an anonymous namespace) in
order to trigger unused function warnings, then removed the unused
functions.

Bug: 209572241
Test: Builds
Change-Id: I549705fec917de763bad1e935c3cad885ab002d5
Merged-In: I549705fec917de763bad1e935c3cad885ab002d5
6 files changed