imap-send: drop unused parameter from imap_cmd_cb callback

imap-send: drop unused parameter from imap_cmd_cb callback

There's a generic callback mechanism for handling plus-continuation of
IMAP commands. It takes the imap_cmd struct itself as an argument. That
seems reasonable, and in a larger imap-using program it might be used.
But in imap-send, we have only one such callback (auth_cram_md5) and it
doesn't use this value, triggering -Wunused-parameter warnings.

We could just mark the parameter as UNUSED. But since this is the only
such function, and because we are not likely to share code with the
upstream isync anymore, we can just simplify the interface to remove
this parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>