🐛 Only render json for manifest requests

We were getting the error:

`ActionController::RespondToMismatchError in
Hyrax::ImagesController#manifest`

`respond_to was called multiple times and matched with conflicting
formats in this action. Please note that you may only call respond_to
and match on a single format per action.`

This commit fixes that by only rendering json for manifest requests.

app/controllers/concerns/hyrax/iiif_av/controller_behavior.rb

@@ -36,10 +36,7 @@ def manifest
 
         json = iiif_manifest_builder.manifest_for(presenter: iiif_manifest_presenter, iiif_manifest_factory: manifest_factory)
 
-        respond_to do |wants|
-          wants.json { render json: json }
-          wants.html { render json: json }
-        end
+        render json: json
       end
 
       private