Remove USB support for tl-mr3020

USB support is removed because of very limited flash.
This allows building for tl-mr3020 again.

USB support has no known use case for this firmware.
Most usb devices would require additional drivers anyway.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>