[BUGFIX] Fix not found classes in 6.x during install

Use the native namespaced class for the calls in ext_localconf and remove
not needed code for master branch

Change-Id: Ic58feec3e11d4f55ad23602d9850972a02163920
Resolves: #53114
Releases: master
Reviewed-on: https://review.typo3.org/27113
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer