dependabot: disable rust checks

As we don't have a Cargo.toml and a Cargo.lock, dependabot for Rust
hasn't been working correctly. Disable, as we now have our own cargo
audit and update workflows.

.github/dependabot.yml

@@ -1,11 +1,5 @@
 version: 2
 updates:
-  - package-ecosystem: "cargo"
-    directory: "/rust"
-    schedule:
-      interval: "daily"
-    commit-message:
-      prefix: "rust:"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: