iwlwifi: remove duplicate iwlagn_mod_params declaration

We only need one declaration, not multiple.
Keep the one in iwl-shared.h, which will
probably be renamed to iwl-modparams.h at
some point in the future.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>