remove extraneous join on jobs table
remove extraneous join on jobs table

this resulted in slowing down the builds table join and forcing it into
a seq scan.

removing this brought the query from ~320ms to ~90ms on our
ci.concourse.ci instance.

concourse/concourse#1218

Signed-off-by: Andrew Edstrom <5434630b00de5ef87566584379f2786462156846@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse