Remove custom SkVertices data and runtime effect `varying` support

This was an experimental feature. It worked (but only the GPU backend).
It was never adopted or used by anyone, to my knowledge. It's a large
amount of code, and a strange corner of SkSL for users to stumble into.

Bug: skia:10680
Change-Id: I0dda0364bce7dbffa58c32de4c7801ec2a6bc42e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/398222
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index bfeef83..448d7e4 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -6,7 +6,9 @@
 
 Milestone 92
 ------------
-  *
+  * Removed custom attributes from SkVertices and the corresponding `varying` feature from
+    SkRuntimeEffect.
+    https://review.skia.org/398222
 
 * * *