Removal of define-obsolete-variable-alias without when

[Po Lu]

Calling `define-obsolete-variable-alias' without a when argument was
obsoleted in Emacs 27, and has been deleted from Emacs 28.

SLIME, or at least the version in NonGNU ELPA, still does this in
hyperspec.el, causing a failure to build in Emacs 28.

According to Stefan Monnier:

> The fix is a small matter of adding the WHEN argument (if they really
> can't figure out what this WHEN should be (like a release version or a
> year), they can just put a nil in there (or remove the thing
> altogether since it's probably a sign that it happened long enough ago
> that it's of no importance any more)).

Thanks.