e1000: cleanup unused prototype

The function e1000_enable_tx_pkt_filtering() was removed in
a previous cleanup patch.  this removes the no longer used
prototype.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>