iwlwifi: remove non-exist extern functions and structures

After re-structure the agn files layout and move all the
agn shared functions out from iwl-5000.c, all those functions and data
structures do not need to be declare "extern" from iwl-5000.c in
iwl-dev.h.

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