lavf: remove unused code

Nothing uses it, and it provides no public API.

Archeological finds:

Commit 101036adb9 added the API.
Commit a8dd8dc6e9 made mpegts.c use it.
Commit af8aae3fa3 disabled it by default in mpegts.c.
Commit ae2bb52cd2 removed all uses of this from mpegts.c.

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

libavformat/mpegts.c

@@ -33,7 +33,6 @@
 #include "mpegts.h"
 #include "internal.h"
 #include "avio_internal.h"
-#include "seek.h"
 #include "mpeg.h"
 #include "isom.h"