MB-7413 Remove duplicated checkpoint logic

Not needed, as same logic is performed in flush_writes/1.

Change-Id: I559369df53e17b6875eb4444a50e186343719ac3
Reviewed-on: http://review.couchbase.org/24013
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>