Update on Rust CI - removing rustc-builds-alt

[pietroalbini]

@SimonSapin

Even if they don’t, as far as Servo is concerned I think only some pre-2017 commits will fail to build without a patch to make the build system download a non-alt toolchain instead.

@josh

delete builds in one month that get zero accesses since now, and wait 6 months to delete alt builds that get any accesses.

Hmm, the infrastructure was configured to periodically delete CI artifacts since 2017, back then 90 days after upload and today 168 days after upload. The artifacts Servo needs to bootstrap those builds are already long gone.