app/appengine: delete cron.sh
app/appengine: delete cron.sh

This script was used on two cron-part.yaml files
which are gone as of CL 46832 and CL 208397.

For golang/go#34744.

Change-Id: If8dd34391e57e011909d22142a4f6c979b8bb51f
Reviewed-on: https://go-review.googlesource.com/c/build/+/336389
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build