HID: i2c-hid: remove i2c_hid_open_mut

Since commit 85ae9113 ("HID: i2c-hid: remove custom locking from
i2c_hid_open/close") there are no more users of i2c_hid_open_mut.
Remove the unused mutex.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Jiri Kosina <jikos@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/i2c-hid/i2c-hid.c

@@ -130,8 +130,6 @@ static const struct i2c_hid_cmd hid_no_cmd =		{ .length = 0 };
  * static const struct i2c_hid_cmd hid_set_protocol_cmd = { I2C_HID_CMD(0x07) };
  */
 
-static DEFINE_MUTEX(i2c_hid_open_mut);
-
 /* The main device structure */
 struct i2c_hid {
 	struct i2c_client	*client;	/* i2c client */