GeneratedSourceFolderManager.removeFolder(boolean) creates deadlock when called from a workspacejob

We have a report at m2e that a job is stuck forever even on cancel:
- https://github.com/eclipse-m2e/m2e-core/issues/1226

From the stack trace it is stuck here:
https://github.com/eclipse-jdt/eclipse.jdt.core/blob/ebbb848546529073e3d6ad8a2bcab0a4e64baae2/org.eclipse.jdt.apt.core/src/org/eclipse/jdt/apt/core/internal/generatedfile/GeneratedSourceFolderManager.java#L504-L512

I guess it does not get the cancel because a null monitor is passed here.