Revert "Use MMAL zero copy on encoder output"

This reverts commit bbb5aa4.

Something odd going on with zero-copy in the test suite. A few images
come out corrupt when it's active on the output port; it's rare (maybe
one in every 500 or so images) but it seems to be repeatable.

picamera/encoders.py

@@ -371,7 +371,6 @@ def start(self, output):
         self.exception = None
         self._open_output(output)
         with self.parent._encoders_lock:
-            self.output_port.params[mmal.MMAL_PARAMETER_ZERO_COPY] = True
             self.output_port.enable(self._callback)
             if self.DEBUG > 0:
                 mo.print_pipeline(self.output_port)