staging: ipx: delete it from the tree

The ipx code moved into the staging tree back in November 2017 and no
one has complained or even noticed it was gone.  Because of that, let's
just delete it.

Note, the ipx header files are not removed here, that will come later
through the networking tree, as that takes a bit more work to unwind.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>