Bug 1009568 - [Messages] Possibly remove the code with forceClean in Thr...

...eadUI.cleanFields r=azasypkin
nit: just for the sake on cleanness - could you please remove "true" parameter from related unit test?
https://github.com/julienw/gaia/blob/397a0e62f65ecca0a3f413fa92b116490f792013/apps/sms/test/unit/activity_handler_test.js#L163