ixgbe: DCB, remove unneeded ixgbe_dcb_txq_to_tc() routine

The ixgbe_dcb_txq_to_tc() routine was used to map TX rings to
a DCB traffic class. Now that a tx_ring has a DCB traffic class
associated with it this routine is no longer needed.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>