usb: gadget: designware-udc: Drop the driver

This driver is not used by any system and is long unmaintained, drop it.
There is a DWC2 OTG driver which is maintained, see CONFIG_USB_GADGET_DWC2_OTG .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>