Remove no longer needed config-workspace header

Summary:
Most of these defines seem to originate from the old kde-workspace;
DPMS is a hard dependency since 7ffb0bb7.
In any case I could not find any trace of any these defines in the
current code.

Test Plan: Built and installed, no apparent difference.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5599