ibmvnic: Remove debugfs support

The debugfs support in the ibmvnic driver is not, and never has been,
supported. Just remove it.

The work done in the debugfs code for the driver was part of the original
spec for the ibmvnic driver. The corresponding support for this from the
server side was never supported and has been dropped.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>