[CLEANUP] Fix faulty type hint in install tool

The DefaultFactory::getStructure method obviously has a wrong
type hint annotated. Besides that, the type checks inside the method
are obsolete, as the types can never change after instantiation.

Releases: master
Resolves: #75401
Change-Id: Id576310aa2b59d715eea520f67d54bc912be434d
Reviewed-on: https://review.typo3.org/47505


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>