MB-10368: Remove couch_db_set module

As we get the sequence numbers through UPR and not from the disk files
the couch_db_set module is obsolete. This commit removes it and all
its traces.

It also removes couch_set_view_util:open_db/2 which is no longer needed.

Change-Id: I9208cc81f1c1146d58ac4d7b8c268ad6986bfad0
Reviewed-on: http://review.couchbase.org/34437
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>