[class.cdtor] Remove duplicate UB label

Author: jensmaurer

source/classes.tex

@@ -6122,7 +6122,7 @@
 indirectly derive from
 \tcode{B}
 shall have started and the destruction of these classes shall not have
-completed, otherwise the conversion results in undefined behavior\ubdef{class.cdtor.convert.or.form.pointer}.
+completed, otherwise the conversion results in undefined behavior.
 To form a pointer to (or access the value of) a direct non-static member of
 an object
 \tcode{obj},