UpdateDocumentListener is added to document but not always removed in
ProjectionViewer, regression introduced by commit
https://github.com/eclipse-platform/eclipse.platform.ui/commit/1848058a52102b5deff39427365e56fe882656b3

This fixes only one possible leak, where a single document was used in
ProjectionViewer, to unblock JDT UI tests.

Fixes https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/2532