write_packet() currently saves the original timestamps of the packet it
got and restores them in case writing fails. This is unnecessary as we
are no longer working directly with the user-supplied AVPacket here; and
it is also pointless because the timestamps may already have been
altered before write_packet().

So remove this and add a general comment to the function that timestamps
may be modified; also remove a long outdated comment about side data.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>