subtitles.h has been included in order to use ff_subtitles_next_line()
to help parsing srt subtitles which at that time had their timing as
part of the payload and not as part of the AVPacket fields. When this
changed (in 55180b32) it has been forgotten to remove this header.

libavcodec/internal.h meanwhile has been added in bb47aa58 and has
never been used at all.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>