cdc-acm: remove unused element of struct acm

write_used was introduced with commit 884b600f ("[PATCH] USB: fix acm
trouble with terminals") but never used since.

Signed-off-by: Tobias Herzog <t-herzog@gmx.de>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>