Drop boolean parameter from AbstractClient::caption

Summary:
With the latest refactoring AbstractClient::captionNormal is the same as
AbstractClient::caption(false) used to be. As there were only two usages
of the false parameter, let's remove this boolean trap and use proper
API calls.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7427