Remove the #sum method from CollectionAssociation

Since edd94cee, CollectionProxy
delegates all calculation methods - except count - to the scope,
which does basically what this method was doing, but since we're
delegating from the proxy, the association method was never called.

activerecord/lib/active_record/associations/collection_association.rb

@@ -161,15 +161,6 @@ def destroy_all
         end
       end
 
-      # Calculate sum using SQL, not Enumerable.
-      def sum(*args)
-        if block_given?
-          scope.sum(*args) { |*block_args| yield(*block_args) }
-        else
-          scope.sum(*args)
-        end
-      end
-
       # Count all records using SQL. If the +:counter_sql+ or +:finder_sql+ option is set for the
       # association, it will be used for the query. Otherwise, construct options and pass them with
       # scope to the target class's +count+.