igc: Remove obsolete DMA coalescing code

DMA coalescing is not applicable for i225 parts. 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>