[TASK] Remove legacy CSH label for impexp

For 4.5 a label for import/export was overridden and an extra file
was created for handling the legacy label.

The label is now merged into the original file and the overriding
option is removed.

Resolves: #76039
Releases: master
Change-Id: I9b2ad90cf2687a72041c1d3f38400fb689b59760
Reviewed-on: https://review.typo3.org/48003


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>