[!!!][TASK] Remove legacy handling of pre 6.0 imports in impexp

The patch removes the legacy file and charset handling from
EXT:impexp when importing a t3d file.

Importing files from pre-6.0 installations within master / 8.0 will
lead to unexpected behaviour.

Resolves: #75829
Releases: master
Change-Id: Ibe904bb1a55f6aa584b58a3261f1108b61514eb8
Reviewed-on: https://review.typo3.org/47831


Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>