Skip meaningless publisher.set_components() call

"parser" component is already passed on initialization.  So it is
not needed to call `set_components()` method.

sphinx/io.py

@@ -307,7 +307,6 @@ def read_doc(app, env, filename):
                     writer=SphinxDummyWriter(),
                     source_class=SphinxDummySourceClass,
                     destination=NullOutput())
-    pub.set_components(None, 'restructuredtext', None)
     pub.process_programmatic_settings(None, env.settings, None)
     pub.set_source(source, filename)
     pub.publish()