iwlwifi: remove unused HT configuration entry tx_chan_width

The tx_chan_width entry is never used, supported_chan_width is used instead.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>