iwlwifi: remove some unused extern declarations from iwl-config.h

After the refactoring, a few extern declarations were accidentally
left in the iwl-config.h file.  Remove them

Change-Id: I79745e440ed5a0a90db61b0daaae374ecef09e86
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>