nleobject: stop using media-duration-factor

This property is not needed. Any internal time effect that an nleoperation wraps is itself responsible for converting seek/segment timestamps.

Previously, the ghostpads were performing a rate conversion after the rate element had already done so, essentially doubling their effect on seeks and segment times. This was always unnecessary, but went unnoticed by the tempochange test because it was using an identity element rather than an actual rate-changing element.