Remove the optimization for std::pair with a key reference. It'll be too much hassle to get a very unusual use case to work on all compilers.

[SVN r56461]

libs/unordered

@@ -1 +1 @@
-Subproject commit 31cd8f4e1645a8ed644b50a6ab45fb55b847fab4
+Subproject commit 2b8680d2c459fc648d9c5ce5526dae9655983bc4