wireless: wext: remove ndo_do_ioctl fallback

There are no longer any drivers (in the tree proper, I didn't
check all the staging drivers) that take WEXT ioctls through
this API, the only remaining ones that even have ndo_do_ioctl
are using it only for private ioctls.

Therefore, we can remove this call.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>