Revert "build index in system.user on insert SERVER-3572"

This reverts commit 3733a988c5.

Currently only builds index if system.users already exists so doesn't work for
first user. Reverting since this is breaking buildbot.

db/security.cpp

@@ -79,7 +79,9 @@ namespace mongo {
             pwd = internalSecurity.pwd;
         }
         else {
+            // static BSONObj userPattern = fromjson("{\"user\":1}");
             string systemUsers = dbname + ".system.users";
+            // OCCASIONALLY Helpers::ensureIndex(systemUsers.c_str(), userPattern, false, "user_1");
             {
                 BSONObjBuilder b;
                 b << "user" << user;