openh264enc: fix broken header AU emission by base class

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: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2178>