usb: dwc2: gadget: do not call usb_gadget_unregister_driver()

that call is completely unnecessary because
usb_del_gadget_udc() already makes sure the
gadget driver is properly unregistered from
the UDC.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>