Do not unload plugins in PluginManager dtor.

According to qt documentation this is not needed. Not unloading plugins
avoids timing issues at destruction time: Plugin related instances can
be deleted after the deletion of MarbleModel now. Previously
MarbleModel deleted the PluginManager, which unloaded the plugins and
then created dangling pointers for plugin related instances not cleaned
up at that time.

Should be backported to the KDE/4.8 branch after testing.

CCBUG: 274558
CCBUG: 288542