Rewrite the last usage of it in itip-formatter.c to use EAttachments
instead.  This also allowed me to kill mail_save_part() in mail-ops.c.

I may need to reevaluate the EAttachment API at some point for all these
fringe EAttachment uses we're accumulating.  Having to asynchronously
"load" an EAttachment whose content is already in memory kinda sucks.