The no-output mode (guarded by AV_CODEC_FLAG2_NO_OUTPUT)
does not provide a noteworthy speedup; in fact, it even
turned out to be slower than the code with the no-output
code removed (ordinary encode: 153259721 decicycles,
noout encode: 153259721; encode with this patch applied:
152451581 decicycles; timings are for encode_frame callbacks
when encoding a 1080p sample to MPEG-4).

(Furthermore, this code was broken for most of its existence
(since 9207dc3b) and no one
noticed, so the no-output mode is probably not used at all.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>