iwlwifi: remove sw_csum_tx

This was a hack done to test the data path of devices that
didn't support well CSUM offload in Tx. This is not needed
anymore.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20201209231352.6c9fc9fb48d5.I2aaebf90e6fe81860105d049a8d35746fa8d86c2@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>