refs #16982 - remove User.current deassignment (no such user)

Allows the scope change to be reverted, as User.current is no longer set
to `nil` (there is no 'admin' user).

This was relying on a bug in Ruby on Rails 4.2 where the `unscoped` call
filtered through thread variables into Subnet.subnet_for which calls
Subnet.all. This is fixed in 5.0, so the user must be set correctly.