imap-send.c: remove unused field imap_store::uidvalidity

I suspect that the existence of both imap_store::uidvalidity and
store::uidvalidity was an accident.

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>

imap-send.c

@@ -124,7 +124,6 @@ struct imap {
 
 struct imap_store {
 	struct store gen;
-	int uidvalidity;
 	struct imap *imap;
 	const char *prefix;
 };