igb: Drop unnecessary checks in transmit path

This patch drops several checks that we dropped from ixgbe some ago.  It
should not be possible for us to be called with either of the conditional
statements returning true so we can just drop them from the hot-path.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>