util: Remove util/cloudbuild
util: Remove util/cloudbuild

The "util/cloudbuild" directory was used to build Docker images on
the Google Cloud Build infrastructure. The gem5 project no longer uses
this service to build Docker images and, as such, this directory (and
its contents) are no longer required.

Change-Id: I724c50ec6045ab95f6b94533f2fd54980230789c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57431
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5