lavf: remove unused code

Nothing uses it, and it provides no public API.

Archeological finds:

Commit 101036ad added the API.
Commit a8dd8dc6 made mpegts.c use it.
Commit af8aae3f disabled it by default in mpegts.c.
Commit ae2bb52c removed all uses of this from mpegts.c.

Signed-off-by: Anton Khirnov <anton@khirnov.net>