Delete removed-at-birth Slice APIs

These APIs never shipped in a final SDK, but were marked @removed
instead of actually removed (to maintain compatibility with DPs).

It's been a while, so just delete them now.

- added in ag/3471192, ag/3341989, ag/3181753, ag/3463807 and ag/3345779
- @removed in ag/3881834

Bug: 155877914
Test: m
Change-Id: Id0775f9de7354860fb6ee39a7cf6f173834c2984