cifs: Remove unused is_server_using_iface()

The last use of is_server_using_iface() was removed in 2022 by
commit aa45dadd ("cifs: change iface_list from array to sorted linked
list")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Steve French <stfrench@microsoft.com>