This can be a side effect of another setting that sets code folding for a plugin that you don't have.
See bug 153449 for an example (written while Mylyn was called Mylar):
, , / , , . , , , .
, :
/instance/org.eclipse.jdt.ui/editor_folding_provider=org.eclipse.mylar.java.ui.editor.foldingprovider
Mylar. , . , .