Removed unnecessary member-signin-urls serializer

refs: https://github.com/TryGhost/Toolbox/issues/245

-  we don't need this serializer because the default serializer will do the same thing

core/server/api/canary/utils/serializers/output/index.js

@@ -73,10 +73,6 @@ module.exports = {
         return require('./tiers');
     },
 
-    get member_signin_urls() {
-        return require('./member-signin-urls');
-    },
-
     get images() {
         return require('./images');
     },

core/server/api/canary/utils/serializers/output/member-signin-urls.js

@@ -1,7 +0,0 @@
-module.exports = {
-    read(data, apiConfig, frame) {
-        frame.response = {
-            member_signin_urls: [data]
-        };
-    }
-};