[TASK] Move t3lib_extMgm alias to compatibility6 extension

ext_autoload.php is ignored now, so the issues why we kept
this alias in the core is obsolete.

Releases: master
Resolves: #67966
Change-Id: Ibdcf362b5c31e938941b374aff786d81da4668ba
Reviewed-on: http://review.typo3.org/40969


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>