Omit unnecessary JSON fields in response

Don't return predecessorid, parentid and ttl fields in response.
predecessorid and parentid are deprecated [1], ttl is not supposed to be
returned by the Sync server [2].

[1] https://docs.services.mozilla.com/storage/apis-1.1.html#id5
[2]
http://hg.mozilla.org/mozilla-central/file/632e3a94bd22/mobile/android/base/sync/CryptoRecord.java#l168