Remove default button on kwinrules KCM

Summary:
The "default" button was behaving the same as "reset" which was unnecessary and confusing.

As the only settings are the list of rules, the only meaning "default" could have is to clear all the rules, which is probably not what the user would want. So I think the best course of action is to remove the button.

Reviewers: zzag, meven, crossi, bport, ervin

Reviewed By: zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28071