iwlwifi: iwl-tx.c cleanup (remove unused parameter and unused local variable).

This patch removes unused parameter and unused local variable in
methods in iwl-tx.c:

- Remove a parameter (is_unicast) from iwl_tx_cmd_build_basic().

- Remove an unused variable name unicast from iwl_tx_skb().

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>