Removed unnecessary webhooks 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

@@ -41,10 +41,6 @@ module.exports = {
         return require('./schedules');
     },
 
-    get webhooks() {
-        return require('./webhooks');
-    },
-
     get posts() {
         return require('./posts');
     },

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

@@ -1,15 +0,0 @@
-const debug = require('@tryghost/debug')('api:canary:utils:serializers:output:webhooks');
-
-module.exports = {
-    all(models, apiConfig, frame) {
-        debug('all');
-        // CASE: e.g. destroy returns null
-        if (!models) {
-            return;
-        }
-
-        frame.response = {
-            webhooks: [models.toJSON(frame.options)]
-        };
-    }
-};