usb: gadget: f_acm: remove empty function

The significant part of this function was removed in 90f7976 ("USB:
Remove unsupported usb gadget drivers"). I would move this to function
bind time but I don't see the point in moving an empty function.
Therefore bye bye.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>