ChangeNotes: Remove support for readOnlyUntil

This was used only by the primary storage migration code, which is now
gone. No additional data migration is needed:
 * Unknown footers in NoteDb commits are silently ignored.
 * Deserializing protos persisted in the ChangeNotesCache will ignore
   the unknown fields.

Change-Id: I45b5736efa3d43584560ea93aeaefe3ea4fd4e80

proto/cache.proto

diff --git a/proto/cache.proto b/proto/cache.proto
index c2ac0d9..52f5e1c 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -181,8 +181,8 @@
   // JSON produced from com.google.gerrit.reviewdb.client.Comment.
   repeated string published_comment = 16;
 
-  int64 read_only_until = 17;
-  bool has_read_only_until = 18;
+  reserved 17;  // read_only_until
+  reserved 18;  // has_read_only_until
 }