Use libetpan's xgmmsgid extension for parsing X-GM-MSGID, rather
than our own code.

Partially revert "Add X-GM-MSGID parsing."  The only bit that we keep
is struct mailmessage's msg_gm_msgid, which we've added to make it
easy for us to pass the result through MailCore.
This reverts commit 4cf14aba8fba72c4985ad7819b381a39556ff758.

Revert "Copy gm_msgid into mailimap_msg_att_static."
This reverts commit 596ad4e632ec6ee5115d86ad6220ca7e570f4edd.