tests,util: Remove Clang Versions <6 from Docker Build
tests,util: Remove Clang Versions <6 from Docker Build

cloud_create_image.yaml is used by Google Cloud build to build the
docker images for the gem5 project. We no longer support Clang versions
<6: https://gem5-review.googlesource.com/c/public/gem5/+/45621. As such
we no longer need to build the image for Clang versions 3.9, 4, and 5.
These were primarily used for the compiler tests, which have been
updated to only test Clang versions >=6.

Change-Id: I0c41cf0f189b4e24976f19a519018f3c9e87bb90
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45680
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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