igc: Remove unused flags

Transmit underrun, late and excess collision flags not in use.
This patch comes to clean up these flags.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>