Removed redundant JSON encode/decode from RCTDataManager

Summary:
@public

For some reason we were manually JSON-encoding the RCTDataManager responses, and then decoding them again on the JS side. Since all data sent over the bridge is JSON-encoded anyway, this is pretty pointless.

Test Plan:
* Test Movies app in OSS, which uses RCTDataManager
* Test any code that uses RKHTTPQueryGenericExecutor to make network requests (e.g. Groups)
* Test the Groups photo upload feature, which uses RKHTTPQueryWithImageUploadExecutor

Libraries/Network/XMLHttpRequest.ios.js

@@ -30,10 +30,7 @@ class XMLHttpRequest extends XMLHttpRequestBase {
       },
       // TODO: Do we need this? is it used anywhere?
       'h' + crc32(method + '|' + url + '|' + data),
-      (result) => {
-        result = JSON.parse(result);
-        this.callback(result.status, result.responseHeaders, result.responseText);
-      }
+      this.callback.bind(this)
     );
   }