rtph264pay: Clean up _payload_nal
rtph264pay: Clean up _payload_nal

Move determining whether we need to fragment at all into the fragmenter.

GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good