ixgbe: remove residual code left over from earlier combining of TXDCTL

Missed some code that was left floating around in the DCB configuration
for the TXDCTL register.  As a result the register was being messed with in
two different spots when we only needed to do the change once.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>