[algs] Remove pair copy constructor

Use default.
diff --git a/src/test-algs.cc b/src/test-algs.cc
index cef2b51..c4610d9 100644
--- a/src/test-algs.cc
+++ b/src/test-algs.cc
@@ -75,6 +75,7 @@
 
   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;
 }