(PUP-3129) Do not store _timestamp fact in immutable hash

The _timestamp fact causes problems when turning on immutable facts
because it is an unsupported data type (Time) that should not leak into
the puppet language. (The system halts with an error if you try prior to
this fix.)
Merge pull request #3010 from kylog/issue/pup-3129/delete-_timestamp-from-immutable-facts-hash

(PUP-3129) Do not store _timestamp fact in immutable hash