Revert "Update corefx"

This reverts commit 2a2c317d07e389088737b391e97853fdb6dbc390.

diff --git a/external/corefx b/external/corefx
index 66a527ffabdce7a994ff8c4748d8445ba7633db3..63c51e726292149b4868db71baa883e5ad173766 160000 (submodule)
--- a/external/corefx
+++ b/external/corefx
@@ -1 +1 @@
-Subproject commit 66a527ffabdce7a994ff8c4748d8445ba7633db3
+Subproject commit 63c51e726292149b4868db71baa883e5ad173766