iwlwifi: remove unused or twice defined members in iwl_priv

This patch removes user_txpower_limit and max_channel_txpower_limit
and use tx_power_user_lmt and tx_power_channel_lmt instead
call_post_assoc_from_beacon is not used

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>