JCBC-668: Remove empty() factory methods on Documents.

Motivation
----------
Recently a (valid) check has been added which makes sure
that every Document has a valid ID. This renders the
empty() factory methods useless, since their usage will
faill imediately.

Modifications
-------------
Remove the empty() factory methods to prevent their usage
and failure at runtime.

Result
------
Preventing incorrect API usage and removing dead methods.

Change-Id: I470ce5d9d84cb0702f3ffd58447a88b8d9eb6b6c
Reviewed-on: http://review.couchbase.org/44705
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>