Revert "iwlwifi: fix build error for CONFIG_IWLAGN=n"

This reverts commit 76232ebf.

iwl_good_ack_health only applies to aggregation and thus only to iwlagn. We
can thus move the function to iwlagn instead of modifying compile flags.
This is done in following patch.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>