imap-send.c: use struct imap_store instead of struct store

In fact, all struct store instances are upcasts of struct imap_store
anyway, so stop making the distinction.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>