[fix] Fix MemorySubobject warning

Author: misonijnik

lib/Core/ExecutionState.h

@@ -230,7 +230,6 @@ struct MemorySubobject {
   unsigned size;
   explicit MemorySubobject(ref<Expr> address, unsigned size)
       : address(address), size(size) {}
-  MemorySubobject &operator=(const MemorySubobject &other) = default;
 };
 
 struct MemorySubobjectHash {