Revert change #31489.

Revert change #31489.
That change was adding a hook to cope with the case when one was
undef'ining *ISA globs, in order to clean up correctly. However, this
broke the case where one was assiging an array ref to @ISA, which
is likely to be more common.
Conclusion: don't undef *ISA. (or more generally don't undef globs
that contain magical variables)
p4raw-link: @31489 on //depot/perl: 5be5c7a687aa37f2ea9dec7988eb57cad1f1ec24

p4raw-id: //depot/perl@31502