Delete @removed-at-birth ImageDecoder 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.

Removed in CLs ag/3709874, ag/3818631, ag/3780966, ag/3811428,
ag/3845199 ag/3742654.

Bug: 155877914
Test: m
Change-Id: I874f3a9706e40bb8b8b41906eb8831c06349dcd5