staging: r8188eu: Remove unused nat25_handle_frame()

Remove nat25_handle_frame() because it is not called by any other
function of the driver.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20210816160617.11949-2-fmdefrancesco@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>