cmd/bent: cleanup cronjob.sh and cronjob configuration toml

Change-Id: I3365065d5fb1408bf5606c676e86e4dc0742f067
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/432360
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>