remove code now that loopers are dead (w.r.t. canvas and paint)

We can't remove the loopers themselves, as they are still used
by android and chrome (they just don't ever pass them to skia).

Eventually each of those clients will resolve this, but for now
we just keep the classes (and tests) in skia.

Bug: skia:4783
Change-Id: I5f507e6bb82280f2bc7c0b21eebe59c287aa9265
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230579
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index e3a2697..24a4fa5 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -7,3 +7,6 @@
 Milestone 78
 
  * Added RELEASE_NOTES.txt file
+
+SkDrawLooper is no longer supported in SkPaint or SkCanvas.
+