libwebm/README.libaom: correct local changes

the addition of V_AV1 was removed in the last roll as it was incomplete
and the constant unused in this code base. webmdec/enc handle
specifying this value.

Change-Id: I30f96e0a5ac7d0e2706b0a1fcab8ac3c8dab5a43
diff --git a/third_party/libwebm/README.libaom b/third_party/libwebm/README.libaom
index 7b99f9f..7ef6c62 100644
--- a/third_party/libwebm/README.libaom
+++ b/third_party/libwebm/README.libaom
@@ -7,4 +7,4 @@
 libwebm is used to handle WebM container I/O.
 
 Local Changes:
-* Add V_AV1 codec id
+* <none>