according to https://bugs.eclipse.org/bugs/show_bug.cgi?id=544977#c21 the
scenario of using the ManifestBuilder without setting the PluginNature is
not a supported scenario. Besides, it doesn't help to get rid of the new
Eclipse warning regarding non-exported packages.
This reverts commit 063484e29f.
other branches that would require redundant updates of the IDE configs.
Finishing the Oxygen update by doing a cleanup will help to reduce the
effort needed to keep the IDE files up to date.