GIF: Internal cleanup - remove color map parameter

GIF: Internal cleanup - remove color map parameter

SkGIFFrameContext::decode() and SkGIFLZWContext::prepareToDecode() do
not need (or use) the global color map, so stop passing it as a
parameter. The parameter was used prior to
https://skia-review.googlesource.com/c/4379/ (different issue!), but we
overlooked removing it then.

Change-Id: I0f477e9db11f7650938d6b868baef69e3b37d86b
Reviewed-on: https://skia-review.googlesource.com/5609
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>