Use removeOne when uninstalling an InputEventFilter

Summary:
Adjusting to a difference from introducing the InputEventSpy. A filter
may only be installed once. Thus removeOne is totally sufficient.

Added a Q_ASSERT to ensure that it is not installed when installing.

Reviewers: KWin, Plasma, mart

Reviewed By: mart

Subscribers: mart, plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3878