Use old define for gif (for now)

Need to update Flutter (to register some codecs explicitly
and/or use the new define name).

We can also delete some now-dead code that depends on
the very old gif decoder (that was not WUFFS).

Change-Id: I4c9b0c1926ea285a9d88229009097e7e8b61eede
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/811823
Reviewed-by: Ben Wagner <bungeman@google.com>
3 files changed