remove unused ruleset in rust repo

[marcoieni]

We don't need this ruleset anymore because we replaced the old update mechanism that used it with Renovate. See rust-lang/infra-team#293

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/rust':
      Rulesets:
          Deleting 'cargo_update'