fix(libutils): make ref move assignable/constructible

src/libutil/ref.hh

@@ -18,11 +18,6 @@ private:
     std::shared_ptr<T> p;
 
 public:
-
-    ref(const ref<T> & r)
-        : p(r.p)
-    { }
-
     explicit ref(const std::shared_ptr<T> & p)
         : p(p)
     {
@@ -75,8 +70,6 @@ public:
         return ref<T2>((std::shared_ptr<T2>) p);
     }
 
-    ref<T> & operator=(ref<T> const & rhs) = default;
-
     bool operator == (const ref<T> & other) const
     {
         return p == other.p;