Do not send out emails for out of disk

We rely now on notifications. We should hover send emails about critical
new notifications. Lets make the admin go to the dashboard to check the
situation.

src/notifications.js

@@ -296,7 +296,6 @@ function diskSpaceWarning(message) {
     assert.strictEqual(typeof message, 'string');
 
     actionForAllAdmins([], function (admin, callback) {
-        mailer.outOfDiskSpace(admin.email, message);
         upsert(admin.id, null, 'Out of Disk Space', message, '/#/graphs', callback);
     }, function (error) {
         if (error) console.error(error);