nleobject: stop using media-duration-factor

nleobject: stop using media-duration-factor

The property had been deprecated and is unused.

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.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/160>