[TASK] Remove extRelPath from module configuration

The module configuration sets the extRelPath but is never evaluated
anymore since 8.0 and the streamlined ModuleLoader functionality.

Resolves: #77058
Releases: master
Change-Id: Iea16d5cf493bd7e584722d494104a515cbc01cf6
Reviewed-on: https://review.typo3.org/48919


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>