[BUGFIX] Don't preserve piVars in felogin links

Change-Id: Icb60d6ab5725da2ffe5f67c2b0dcf02280a91ab3
Releases: 6.1, 6.0, 4.7
Fixes: #46829
Reviewed-on: https://review.typo3.org/20350
Reviewed-by: Markus Klein
Reviewed-by: Michael Schams
Tested-by: Michael Schams
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters