[!!!][TASK] Remove ModuleLoader->getRelativePath()

The method to fetch the relative path for images inside
the module loader is not used anymore, as the paths
for the icons are handled through the Icon API now.

The method can be removed safely, and was simply forgotten
to be removed with #72405 (see https://review.typo3.org/#/c/45417/).

Resolves: #74029
Releases: master
Change-Id: I0d9708de3df5f22bb551db07050e479aa0071134
Reviewed-on: https://review.typo3.org/47013


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>