iwlwifi: remove unused members of iwl_ht_info

Some members of iwl_ht_info are unused, and one of
them is write-only, so we can remove these three:
max_amsdu_size, ampdu_factor and mpdu_density.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
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>