Deprecate malloc_object

This pointer predicate is never used as the changes in e05d813 made
it redundant.

Author: tautschnig

src/util/pointer_predicates.h

@@ -24,6 +24,7 @@ DEPRECATED(SINCE(2021, 5, 6, "Use object_size instead"))
 exprt dynamic_size(const namespacet &);
 exprt pointer_offset(const exprt &pointer);
 exprt pointer_object(const exprt &pointer);
+DEPRECATED(SINCE(2021, 5, 6, "Unnecessary, remove any use"))
 exprt malloc_object(const exprt &pointer, const namespacet &);
 exprt object_size(const exprt &pointer);
 exprt dynamic_object(const exprt &pointer);