staging: net: remove pc300 driver

To quote the TODO from staging/net/:

  PC300:

  The driver is very broken and cannot work with the current TTY
  layer. It is inevitable to convert it to the new TTY API. If no
  one steps in to adopt the driver, it will be removed in the 3.7
  release.

Nothing has changed since more than _one_ year on this driver, thus
just remove it since we already moved past 3.7. If somebody steps
up and does a whole rework, he/she, of course, is free to resubmit
it. Since this is the only one in the net directory, we can remove
it as well.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>