igc: Remove MSI-X PBA Clear register

MSI-X PBA Clear register 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>