[BUGFIX] Correct order of key / field deletion

Prevent an SQL error when deleting a key and the field belonging
to the key in the install tool at the same time.

Therefore the order of deleting keys and fields is changed from
field first and key second to key first and field second.

Resolves: #50450
Releases: master, 6.2
Change-Id: I30aff6ea6c2bf01dec6197eec8fd3aa7306cfcbb
Reviewed-on: http://review.typo3.org/34101
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>