bpo-35065: Remove StreamReaderProtocol._untrack_reader (#10212)

The call to `_untrack_reader` is performed too soon, causing the protocol
to forget about the reader before `connection_lost` can run and feed the
EOF to the reader. See bpo-35065.

Lib/asyncio/subprocess.py

@@ -36,11 +36,6 @@ class SubprocessStreamProtocol(streams.FlowControlMixin,
             info.append(f'stderr={self.stderr!r}')
         return '<{}>'.format(' '.join(info))
 
-    def _untrack_reader(self):
-        # StreamWriter.close() expects the protocol
-        # to have this method defined.
-        pass
-
     def connection_made(self, transport):
         self._transport = transport