Deprecate add_constituent

This was only used by dor-utils and we've stopped using it there:
sul-dlss-deprecated/dor-utils#41

lib/dor/models/concerns/contentable.rb

@@ -185,12 +185,12 @@ def decommission(tag)
       TagService.add self, "Decommissioned : #{tag}"
     end
 
-    # TODO: Move to Dor-Utils.
     # Adds a RELS-EXT constituent relationship to the given druid
     # @param [String] druid the parent druid of the constituent relationship
     #   e.g.: <fedora:isConstituentOf rdf:resource="info:fedora/druid:hj097bm8879" />
     def add_constituent(druid)
       add_relationship :is_constituent_of, ActiveFedora::Base.find(druid)
     end
+    deprecation_deprecate add_constituent: 'Use add_relationship :is_constituent_of instead'
   end
 end