imap-send: drop unused fields from imap_cmd_cb

imap-send: drop unused fields from imap_cmd_cb

The imap_cmd_cb struct has several fields which are totally unused.
Presumably they did useful things in the upstream isync code from which
this is derived, but they don't in our more limited program. This is
particularly confusing for the "done" callback, which (as of the
previous patch) no longer matches the signature of the adjacent "cont"
callback.

Since we're unlikely to share code with isync going forward, we should
feel free to simplify the code here. Note that "done" is examined but
never set, so we can also drop a little bit of code outside of the
struct definition.

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