[BUGFIX] Stop using the BigDocument class.

This class is deprecated.

Resolves: #63457

Change-Id: Ia48c7785c42db9863380fd3d1afb041d39c78f1c
Reviewed-on: http://review.typo3.org/34879
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>