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>