STYLE: add const_cast when deleting pointers in objectRegistry

- makes the intention clearer, allows for future tightening of
  iterator access rules.

src/OpenFOAM/db/objectRegistry/objectRegistry.C

@@ -370,7 +370,7 @@ bool Foam::objectRegistry::erase(const iterator& iter)
 
     if (iter.found())
     {
-        regIOobject* ptr = iter.val();
+        regIOobject* ptr = const_cast<iterator&>(iter).val();
 
         const bool ok = HashTable<regIOobject*>::erase(iter);