Remove email host variable

Today I did a test run if I can use the mail.gfz-potsdam.de server from the django app in the central cv. Unfortunaly this is not possible at the moment, at least not with the current configuration (as the server is not at the GFZ anymore).

In order to avoid errors when the users may make suggestions in the CV, I would say that we remove this variable from the config for the moment (until I find out what to do exactly in order to make it work).