iwlwifi: remove remaining software checksum code

After the removal of the software checksum code for the
A-MSDU path that we had for testing, the csum_skb variable
stuck around. Remove it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210411124417.280f268ae679.Iad455b6c91e427c9f74963bbd3eb0ce743aaac53@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>