Removed unnecessary emails serializer

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

-  we don't need this serializer because the default serializer will call the emails  mapper

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

@@ -1,22 +0,0 @@
-const mappers = require('./mappers');
-
-module.exports = {
-    read(email, apiConfig, frame) {
-        frame.response = {
-            emails: [mappers.emails(email, frame)]
-        };
-    },
-
-    browse(page, apiConfig, frame) {
-        const data = {
-            emails: page.data.map(model => mappers.emails(model, frame)),
-            meta: page.meta
-        };
-
-        frame.response = data;
-    },
-
-    get retry() {
-        return this.read;
-    }
-};

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

@@ -125,10 +125,6 @@ module.exports = {
         return require('./email-preview');
     },
 
-    get emails() {
-        return require('./emails');
-    },
-
     get snippets() {
         return require('./snippets');
     },