%n will show the unread message count in the mailbox.  %m will show
total message count.  Except for %n with IMAP, these both require
$mail_check_stats to be set, which puts these counts inside BUFFY.

Since the imap_mboxcache is never fresher than the value in BUFFY,
remove the special imap_mailbox_state() call.  Instead, just update
from the current Context for all mailboxes.

Remove the logic that overrode the %N format string to show unread
count for IMAP mailboxes.  If they want to see unread count, they will
have to change the $folder_format.

Although it doesn't look possible to reuse browser_state.entry slots,
change the OP_DELETE_MAILBOX to memset(0) the deleted slot.  Then, we
can change to logic in add_folder() so it doesn't need to zero-out
unset values.