[BUGFIX] ClassLoader not working with NullBackend for legacy classes

Removed unused line that creates a fatal error.
Implemented eval in case NullBackend is set for “cache_core”
and no alias mapping is included.

Resolves: #55517
Releases: 6.2
Change-Id: Ie1d58985496c4561d8b20fedbd030ad98760becd
Reviewed-on: https://review.typo3.org/27218
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn