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/mail_templates/out_of_disk_space.ejs

@@ -1,22 +0,0 @@
-<%if (format === 'text') { %>
-
-Dear <%= cloudronName %> Admin,
-
-your server is running out of disk space.
-
-Disk space logs are attached.
-
--------------------------------------
-
-<%- message %>
-
--------------------------------------
-
-
-Powered by https://cloudron.io
-
-Sent at: <%= new Date().toUTCString() %>
-
-<% } else { %>
-
-<% } %>