chore(ci): remove cargo ecosystem from dependabot config

## What/Why

Remove the cargo package ecosystem from the dependabot configuration per
PR review feedback (EpicGames#21).

## Proof it works

YAML-only change; validated structure manually. Remaining ecosystems
(uv, github-actions, docker) are unchanged.

## Risk + AI role

Low -- config removal only. AI-assisted (Claude Opus 4.6) for the edit.

## Review focus

Confirm cargo removal aligns with upstream maintainer intent.

Signed-off-by: jmeridth <jmeridth@gmail.com>

Author: jmeridth

.github/dependabot.yaml

@@ -1,21 +1,6 @@
 ---
 version: 2
 updates:
-  - package-ecosystem: "cargo"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    cooldown:
-      default-days: 7
-    commit-message:
-      prefix: "chore(deps)"
-    labels: ["rust", "dependencies"]
-    groups:
-      dependencies:
-        applies-to: version-updates
-        update-types:
-          - "minor"
-          - "patch"
   - package-ecosystem: "uv"
     directory: "/"
     schedule: