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.
this way, no patching of the userGroupProvider property after
de-serialization is necessary, and no replacement of a User object in
the UserStore should ever happen.
Change-Id: I23a8ae53b6a53ca89a2a0b8a11dc779f4034db57