Consider adding MOZ_SERVICES_HEALTHREPORT to mozconfig

Someone suggested on the blog that we unset MOZ_SERVICES_HEALTHREPORT in mozconfig: https://blog.torproject.org/comment/285469#comment-285469

Apparently unsetting this would avoid including the file toolkit/components/telemetry/healthreport-prefs.js, which currently contains:

pref("datareporting.healthreport.infoURL", "https://www.mozilla.org/legal/privacy/firefox.html#health-report");
pref("datareporting.healthreport.uploadEnabled", true);

We already set datareporting.healthreport.uploadEnabled to false, so this wouldn't fix any issue, but would be defense in depth.