BUG: Fixed 0003876: All python modules fail after uninstalling an extension

Wrong PYTHONPATH setting was set in wrong format in qSlicerExtensionsManagerModelPrivate::removeExtensionPathFromLauncherSettings.
Fixed by updating the setting the same way as in qSlicerExtensionsManagerModelPrivate::addExtensionPathToLauncherSettings.

http://na-mic.org/Mantis/bug_view_advanced_page.php?bug_id=3876

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24140 3bd1e089-480b-0410-8dfb-8563597acbee