pp.c:pp_undef: Don’t vivify the scalar slot

When PERL_DONT_CREATE_GVSV is defined, perl generally does not vivify
the scalar slot in every GV.  But it hides that implementation detail
by vivifying it when *foo{SCALAR} is accessed.

undef(*foo) was one exception to this.  It vivified the scalar in
the scalar slot regardless of whether PERL_DONT_CREATE_GVSV was
defined.

Until recently, it was not safe to remove this exception, because a
typeglob with no scalar could be a candidate for downgrading (see
gv.c:gv_try_downgrade), causing global pointers like PL_DBgv to point
to freed SVs.  Recent commits have fixed all those cases, so this
is now safe.

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