Fix caller of getGuaranteedNurseryRange() and use new functions

[dmitripivkine]

getGuaranteedNurseryRange() in GCExtensionsBase is defind always no. There is no need to check J9VM_GC_GENERATIONAL (and technically this is wrong flag any way, OMR_GC_MODRON_SCAVENGER should be checked instead). This function is going to set output parameters to NULL if OMR_GC_MODRON_SCAVENGER is not defined

#depends https://github.com/eclipse/omr/pull/6715

Signed-off-by: Dmitri Pivkine [email protected]