i40e: Drop dead code and flags from Tx hotpath

Drop Tx flag and TXSW which is tested but never set.

As a result of this change we can drop a complicated check that always
resulted in the final result of i40e_tx_csum being equal to the
CHECKSUM_PARTIAL value.  As such we can replace the entire function call
with just a check for skb->summed == CHECKSUM_PARTIAL.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>