Re-allow running Dolphin as the root user (but still not using sudo)

Summary:
Prohibiting the use of Dolphin as the actual root user (not using `sudo` or `kdesu`) breaks legitimate use cases for using the root user. An example is Kali, a distro that logs in as the root user by default as a deliberate design choice.

In such an environment, there is no additional security vulnerability beyond what you're already potentially exposing yourself to. So, let's re-enable it.

BUG: 387974
FIXED-IN: 18.08.0

Test Plan:
- Log in as normal user and run `sudo dolphin`: you get an error message.
- Log in as normal user and run `kdesu dolphin`: you get an error message.
- Log in as the root user and run dolphin normally: it works.

Reviewers: markg, elvisangelaccio, #dolphin

Reviewed By: markg

Subscribers: chinmoyr, cfeck, elvisangelaccio, mmustac, Fuchs, markg, graesslin, nicolasfella, zzag, kfm-devel, emmanuelp

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D12795