This effectively reverts recent commit d70e2fb3 (from 15th Aug).
The use of parser seems to be a problem for UG workflow, in the end -
the point is that it may cache a packet until arival of next packet
(commit 6e9a4142), so in that commit, as a solution flushing with EOF
was added.
However, it seems to produce problems with simple (but not with H.264 parser):
```
uv -d gl -t testcard -c libavcodec:encoder=libx265
```
because the parser seems to be confused when parsing frames after EOF
so for the subsequent frames it consumes 1 byte producing 1 byte output. This
is mostly harmless (it is actually '\0', part of the start code), but
it produces errors:
```
[lavc hevc @ 0x68216c0055c0] missing picture in access unit with size 1
```
Possible solution would be to re-create the parser for every frame
(sic!) but it is unclear the overhead (which applies also to parsing the
frames, anyways). Anyways, as piggy-backed frames should not occur since
the commit c57f2fc5, it is perhaps best to remove this stuff altogether.
e2f0bb34d7