Removed extraneous permission flag in member signin urls

- This doesn't affect permissions, only permissions inside an endpoint config block does that
- Rather it creates an extra unused "endpoint" called member_signin_urls.permissions
- same as https://github.com/TryGhost/Ghost/commit/b53296c4d

core/server/api/canary/memberSigninUrls.js

@@ -8,7 +8,6 @@ const messages = {
 
 module.exports = {
     docName: 'member_signin_urls',
-    permissions: true,
     read: {
         data: [
             'id'