[TASK] Remove saltedpasswords bulk update task

This patch is one step into merging the salted passwords
extension code into the core extension, starting with removing
obsolete stuff: The bulk update task is a use-once-throw-away
task aimed to help upgrading systems older than 10 years.
It is removed now. The patch includes an extensive
important ReST file explaining details and alternatives.

Change-Id: I2e9064438e0ac257e95bb0768860f587d8e29375
Resolves: #85022
Releases: master
Reviewed-on: https://review.typo3.org/56977


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>