Remove String.fromCodePoint shim (#279)

This is not necessary anymore if we drop support for Node.js v0.10 and v0.12.

Ref. https://github.com/babel/babel/issues/4315.

test/fixtures/jsx/basic/entity/expected.json

@@ -136,7 +136,7 @@
                 }
               },
               "extra": null,
-              "value": "💩"
+              "value": "\uD83D\uDCA9"
             }
           ]
         }
@@ -144,4 +144,4 @@
     ],
     "directives": []
   }
-}
+}