remove todo about case insensitive INBOX

this is *our* magic string, not IMAP's.
ok, admittedly, we *also* send it to IMAP, but that's just convenience.
actually making it case insensitive would improve interoperability with
thunderbird (which interprets INBOX even if qualified), but would break
existing setups (including mine).

TODO

@@ -2,8 +2,6 @@ have a look at libpathfinder.
 
 find out why mutt's message size calc is confused.
 
-iNboX is case insensitive!
-
 add some marker about message being already [remotely] trashed.
 real transactions would be certainly not particularly useful ...