The single user of the downstream gpio-cascade driver has been switched
to use the upstream gpio-line-mux driver. Thus, we can drop the former
now.
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22206
Signed-off-by: Robert Marko <robimarko@gmail.com>