vport: Remove unused error types.

vport: Remove unused error types.

We currently track rx_over_errors, rx_crc_errors, rx_frame_errors,
and collisions but never increment these counters.  It seems likely
that we will never use them since they are primarily hardware errors
and we pull hardware stats directly from the NIC.  This removes those
counters, saving 32 bytes per port.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>