[kde-frameworks/kauth] Remove polkit-kde-agent dependency.

KAuth does not in fact depend on polkit-kde-agent for policykit support, and furthermore, having such a dependency doesn't make any sense, as polkit-kde-agent is a KDE4 package, so its dependencies block KF5.

I have tested merging this package with this commit and USE="policykit", and it seems to work as expected.