iwlwifi: Remove inconsistent and redundant declaration

Remove declaration of iwl_alloc_traffic_mem from iwl-agn.h,
from methods that was exposed to support MVM.

MVM doesn't have to use this declaration.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>