Remove deprecated tile filter header

Also removes the now-empty SkImageFilters::RegisterFlattenables()
function (was public, but is private-use only, so no one should be using
it anyways).

The remainder of SkImageFilters.cpp will be cleaned up in a follow-up CL

Bug: skia:11230
Change-Id: Ibdccff70d37e78e935cadcaff2ac90384e27c990
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/372120
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed