[BUGFIX] Use userInput instead of own broken method

The user data from form post submit is broken after install tool
rewrite. Instead of using own method to gather data changed to
the userInput variable that is set by the calling action.

Resolves: #49260
Related: #49162
Related: #49258
Releases: 6.2
Change-Id: I2ce0b5e4bfcb0ef013039297a9186f0f59413b31
Reviewed-on: https://review.typo3.org/21511
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn