Remove APP15-based gainmap support

This never became an official format, and it was a surprise to see that
it hadn't been removed.

Bug: b/338342146
Change-Id: I450eab328e1ff59d54dddb31090c35b4417ce68d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/849336
Commit-Queue: Christopher Cameron <ccameron@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed