[BUGFIX] Allow multiple aliases for a class

When working on #55559 (refactoring class alias handling)
accidentally code was introduced to avoid more than
one alias for a class name.
Remove this code to make multiple aliases work again.

Fixes: #55897
Releases: 6.2
Change-Id: I8b4b721c57c52d117130947fe92c564d6ddcdd9b
Reviewed-on: https://review.typo3.org/27558
Reviewed-by: Claus Due
Tested-by: Claus Due
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring