videodecoder: remove useless code in negotiate_default_caps()

gst_video_decoder_negotiate_default_caps() is meant to pick a default output format when we need one earlier because of an incoming GAP. It tries to use the input caps as a base if available and fallback to a default format (I420 1280x720@30) for the missing fields.

But the framerate and pixel-aspect were not explicitly passed to gst_video_decoder_set_output_state() which is solely relying on the input format as reference to get the framerate anx pixel-aspect-ratio. So there is no need to manually handling those two fields as gst_video_decoder_set_output_state() will already use the ones from upstream if available, and they will be ignored anyway if there are not.

This also prevent confusing debugging output where we claim to use a specific framerate while actually none was set.