usb: gadget: at91_udc: remove unused release function

As the driver call usb_add_gadget_udc --> usb_add_gadget_udc_release
with NULL as release parameter, so it will use usb_udc_no_release.
So, the release in driver won't used, remove it.

And at the same time, in the usb_add_gadget_udc_release will set the
gadget name, so remove it also in driver.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>