Fix a minor parameter issue

Summary: D1595 split encodeJSONForHTTPResponse() into two methods, but left a
straggling $use_javelin_shield parameter which is no longer used.

Test Plan: Caught errors in error log.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1663