Allow default zone description to be changed

Removed the section of code that was blocking the default
zone description to be changed.

https://bugzilla.redhat.com/show_bug.cgi?id=1098305
Should change to sort by the description when building the server_zones array for the form, since that is now how we want it to be displayed:
```ruby
zones = Zone.order('lower(description)')
```
Also remove the .sort in the app/views/shared/views/ems_common/_form.html.erb view, since the array will already be sorted.