Remove the unnecessary LegislativePeriodExtension#people_ids method

Thanks to @tmtmtmtm for pointing out that the people method can be
much simpler, making the people_ids method unnecessary.

lib/everypolitician_extensions.rb

@@ -38,14 +38,8 @@ def memberships_at_end
       end
     end
 
-    def people_ids
-      @people_ids ||= Set.new(memberships.map(&:person_id))
-    end
-
     def people
-      @people ||= legislature.popolo.persons.select do |p|
-        people_ids.include?(p.id)
-      end
+      @people ||= memberships.map(&:person).uniq(&:id)
     end
 
     def top_identifiers