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>

drivers/usb/class/cdc-acm.h

@@ -101,7 +101,6 @@ struct acm {
 	u8 *notification_buffer;			/* to reassemble fragmented notifications */
 	unsigned int nb_index;
 	unsigned int nb_size;
-	int write_used;					/* number of non-empty write buffers */
 	int transmitting;
 	spinlock_t write_lock;
 	struct mutex mutex;