Do we need colorspace in erase?

No one seems to be using the functionality, so can remove it.
For now, adding a few nullptr_t variants to make clients compile
that are explicitly passing null for the colorspace.

Change-Id: I6db4f1dc591bddc0570690250d4149fde6ea9251
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/660560
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed