Remove kwallet4 support

Summary:
Kwallet5 migrates the old wallet and kdelibs4 code can connect to the
kwalletd5 daemon.

I can't envision any situation where a person would need to be compiling
the latest version of this Plasma code without having the KF5 wallet
runtime.

Test Plan: N/A

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20217