igc: Remove unused MDIC_DEST mask

Formally Destination bit should be kept reserved to
support legacy drivers and ignore on write/read
operation
Not applicable for i225 parts

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>