Remove use of deprecated implicit copy/move assignment operators

By removing custom copy constructor.

src/hb-meta.hh

@@ -142,7 +142,6 @@ template <typename T>
 struct hb_reference_wrapper
 {
   hb_reference_wrapper (T v) : v (v) {}
-  hb_reference_wrapper (const hb_reference_wrapper& o) : v (o.v) {}
   bool operator == (const hb_reference_wrapper& o) const { return v == o.v; }
   bool operator != (const hb_reference_wrapper& o) const { return v != o.v; }
   operator T () const { return v; }
@@ -153,7 +152,6 @@ template <typename T>
 struct hb_reference_wrapper<T&>
 {
   hb_reference_wrapper (T& v) : v (hb_addressof (v)) {}
-  hb_reference_wrapper (const hb_reference_wrapper& o) : v (o.v) {}
   bool operator == (const hb_reference_wrapper& o) const { return v == o.v; }
   bool operator != (const hb_reference_wrapper& o) const { return v != o.v; }
   operator T& () const { return *v; }