Drop CONFIGURING file as it's unmaintained for at least a decade

Summary:
It still mentions dcop, so nobody updated it since the introduction of
dbus which is > 10 years. If nobody updates it and nobody complains that
it mentions dcop it means nobody reads it.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17712