usb: core: remove the Kconfig entry for USB_DEBUG

Since we have already removed the usage of CONFIG_USB_DEBUG, it is
meaningless that there is still a configuration entry for CONFIG_USB_DEBUG.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>