imap-send.c: remove namespace fields from struct imap

imap-send.c: remove namespace fields from struct imap

They are unused, and their removal means that a bunch of list-related
infrastructure can be disposed of.

It might be that the "NAMESPACE" response that is now skipped over in
get_cmd_result() should never be sent by the server.  But somebody
would have to check the IMAP protocol and how we interact with the
server to be sure.  So for now I am leaving that branch of the "if"
statement there.

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>