Remove Advanced section from screen config

Summary:
There was only one option hidden behind the andvanced section, just have
one line for refresh rate that is visible.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, andreaska, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14325