ges-launcher: do not set rendering details too early

ges-launcher: do not set rendering details too early

It looks like the _set_rendering_details call is superfluous
in _startup(), as it will get called in run_pipeline.

The problem with calling it before timeline_set_user_options
is that we are going to fail creating a smart profile if
the user selected eg --track-types=video, as the get_smart_profile
method compares the tracks in the asset with those on the timeline.

Reproduce with a video-only clip:

ges-launch-1.0 --track-types=video +clip file://$PWD/jelly.mp4 \
inpoint=15.0 -o foo.mp4 --smart-rendering

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/228>