igc: Remove IGC_MDIC_INT_EN definition

IGC_MDIC_INT_EN definition is not used. This patch comes to tidy up the
driver code.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>