iwlwifi: no chain noise support for 6x50 series

For initial bring up of 6x50 series NICs, no chain noise support in
uCode, this feature will be added in the later stage of development.

Two chain noise related functions are removed from 6x50 series:
1. gain computation
2. chain noise reset

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>