AK: Remove copy_ref().

This patch removes copy_ref() from RefPtr and NonnullRefPtr. This means that
it's now okay to simply copy these smart pointers instead:

- RefPtr = RefPtr // Okay!
- RefPtr = NonnullRefPtr // Okay!
- NonnullRefPtr = NonnullRefPtr // Okay!
- NonnullRefPtr = RefPtr // Not okay, since RefPtr can be null.

AK/NonnullRefPtr.h

@@ -142,12 +142,6 @@ public:
         return *this;
     }
 
-    CALLABLE_WHEN(unconsumed)
-    NonnullRefPtr copy_ref() const
-    {
-        return NonnullRefPtr(*m_ptr);
-    }
-
     CALLABLE_WHEN(unconsumed)
     SET_TYPESTATE(consumed)
     T& leak_ref()

AK/RefPtr.h

@@ -177,11 +177,6 @@ public:
         return *this;
     }
 
-    RefPtr copy_ref() const
-    {
-        return RefPtr(m_ptr);
-    }
-
     void clear()
     {
         deref_if_not_null(m_ptr);