Remove unused messages and help greppers with comments

According to a simple script (and subsequent manual check), 4 messages
aren't used anywhere, so I'm removing them. Also, add comments with all
variation of messages. Comments are cheap and help when grepping.

Bug: T203871
Change-Id: Ia3e4a5db2d7a0a51287a1d6b1d8e4dfa161b3fb3