Remove the checkpoint extension command

This was part of maintainence mode in 1.8.x and is not longer
supported in the 2.x series of Couchbase.

Change-Id: Iedbc87af2d006915aa811f6eac32c5282673ccc9
Reviewed-on: http://review.couchbase.org/29192
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>