Remove SkShaders::Lerp

It's entirely unused, and trivial for clients to create with SkSL.

Change-Id: I197986232d3706f5af3a197f0fb8e744e1009e5f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/419796
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index e671525..43bdc29 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -9,6 +9,9 @@
   * Removed SkPaint::getHash
     https://review.skia.org/419336
 
+  * Removed SkShaders::Lerp. It was unused (and easy to replicate with SkRuntimeEffect).
+    https://review.skia.org/419796
+
 * * *
 
 Milestone 92