[kstyle] Drop Qt 4 style plugin

Summary:
KWindowSystem cannot be used in the Qt 4 style plugin. On the other hand,
we need to use KWindowSystem to make QWidget-based internal clients in
KWin cast drop-shadows.

Another problem with the Qt 4 style plugin is that some distributions
have already dropped Qt 4, so one has to build it first in order to
verify that his or her change works with Qt 4.

Given maintenance burden and the fact that Qt 6 is around the corner,
this change drops the Qt 4 style plugin.

Reviewers: KWin, Plasma, davidedmundson

Reviewed By: KWin, Plasma, davidedmundson

Subscribers: davidedmundson, mart, plasma-devel

Tags: Plasma

Maniphest Tasks: T12496

Differential Revision: https://phabricator.kde.org/D26476