KateMessageWidget: remove unused event filter installation

Summary:
Left-over from removal of event filter when this code was still part
of Kate repo, commit 487513e4a3573916eff9c417f98896c80c2f6955

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D26704