usb: gadget: drop unused USB_GADGET_MUSB_HDRC

The functionality meant to be represented by this symbol will be
re-added later, but for now, USB_GADGET_MUSB_HDRC is in fact unused and
can be dropped.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>