Show "No messages" when there's no items on message list

Show this when there's no messages, and no "Load more messages" footer.
i.e. Only Outbox, Drafts, and exchange mailboxes can show "No messages".

Also removed an obsolete comment.

Bug 2363624

Change-Id: If483d33cfacb186d0fe54b4f4382333a332be84c
1 file changed