libutil: remove unused ref constructor

this one is dangerous anyway.

Change-Id: Id4fb777fff516cdb6a6e8e0b2fd7611418ae2e49

lix/libutil/ref.hh

@@ -32,13 +32,6 @@ public:
             throw std::invalid_argument("null pointer cast to ref");
     }
 
-    explicit ref<T>(T * p)
-        : p(p)
-    {
-        if (!p)
-            throw std::invalid_argument("null pointer cast to ref");
-    }
-
     T* operator ->() const
     {
         return &*p;