openh264enc: fix broken header AU emission by base class

This encoder advertises alignment=au as output format, which means
each output frame should contain a full decodable access unit.

The video encoder base class is not aware of our output alignment
and will output spurious buffers with just the SPS/PPS inside when
we call gst_video_encoder_set_headers(), which is broken because
each buffer is supposed to contain a full decodable access unit
in our case.

Just don't tell the base class about our headers, they will be
sent at the beginning of each IDR frame anyway.

Part-of: <gstreamer/gst-plugins-bad!2478>
29 jobs for !2478 with backport-mr-2178-into-1.18-openh264-fix-sps-pps-header in 27 minutes and 45 seconds (queued for 11 seconds)
merge request