staging: et131x: Remove et131x driver from drivers/staging

The current version of the et131x driver has been accepted into the
main tree at /drivers/net/ethernet, so it can now be removed from
staging.

The MAINTAINERS entry has not been touched here, as the patch to
add the driver to drivers/net modifies it correctly.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>