[!!!][TASK] Drop ext:typo3db_legacy

The extension containing legacy $GLOBALS['TYPO3_DB'] has been moved to
github at https://github.com/FriendsOfTYPO3/typo3db_legacy
and is available as TER extension and now dropped from core.
An upgrade wizard is added to dowload and install the extension
if needed.

Change-Id: Id99794844e39632a3451e19c7a7d55d47bc3118c
Resolves: #81023
Releases: master
Reviewed-on: https://review.typo3.org/52612


Reviewed-by: Johannes Goslar <jogo@kronberger-spiele.de>
Tested-by: Johannes Goslar <jogo@kronberger-spiele.de>
Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>