iwlwifi: remove get_stats callback function

The low level transmission function is performed at uCode layer
for all the "agn" NICs, there is no statistics information available
for mac80211 get_stats() call. Remove the callback function to
avoid misleading information that returned success when indeed it is not
supported. Now return "not supported".

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