smb/client: remove unused rq_iter_size from struct smb_rqst

Reviewed-by: David Howells <dhowells@redhat.com>
Fixes: d08089f6 ("cifs: Change the I/O paths to use an iterator rather than a page list")
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>