bc_http: remove unused 'retry_on_missing' field

Change-Id: Ie23d73b79e4511cc41aa9efddbcbe4a98b1a26e3
Reviewed-on: http://review.couchbase.org/37574
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>