Platform: Remove ComCast RDK

The Comcast RDK platform was contributed years ago, but since then,
nobody has expressed any interest in it, leaving it to the maintainers
to keep it in shape. As it turns out, the maintainers can think of
better ways to spend their time, so let's just drop it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>