usb: gadget: fsl_mxc_udc: Remove the driver

Since 5.10-rc1 i.MX is a devicetree-only platform, and this driver
was only used by the old non-DT i.MX devices.

Remove the driver as it has no users left.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/20201210210413.15262-1-festevam@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>