[algs] Remove pair copy constructor

Use default.

src/hb-algs.hh

@@ -215,7 +215,6 @@ struct hb_pair_t
   typedef hb_pair_t<T1, T2> pair_t;
 
   hb_pair_t (T1 a, T2 b) : first (a), second (b) {}
-  hb_pair_t (const pair_t& o) : first (o.first), second (o.second) {}
 
   template <typename Q1, typename Q2,
 	    hb_enable_if (hb_is_convertible (T1, Q1) &&

src/test-algs.cc

@@ -75,6 +75,7 @@ main (int argc, char **argv)
 
   hb_pair_t<const int*, int> xp = hb_pair_t<int *, long> (nullptr, 0);
   xp = hb_pair_t<int *, double> (nullptr, 1);
+  xp = hb_pair_t<const int*, int> (nullptr, 1);
 
   return 0;
 }