Don't use any default www-user

Otherwise, nipap-www will always automatically create the user test with password test.

Fix for https://github.com/SpriteLink/NIPAP/issues/572
Right, but the sed line we have right now in postinst already adds "@local", so it won't really make a difference to the end result. What it does though is look confusing, IMHO :P 
We are only talking about the default value here, anyone is free to add @local or something else to their configuration.