Revert "mpegtspacketizer: memcmp potentially seen_before data"

Revert "mpegtspacketizer: memcmp potentially seen_before data"

This reverts commit fcad4cc646a23e4e621ec5e8485958ab78d98090.

This was wrong is so many ways.

* The memcmp was badly used (it should use == 0 to check the data is identical,
  and not != 0)
* There was no boundary checks on the present stream section_data when passing
  it to memcmp.
* The return value should have been TRUE (i.e. we have done all checks, none of
  them failed, therefore the section has been seen before)
* stream->section_data would *always* be NULL if the section had already been
  processed

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1559

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3421>