Remove "uv" ecosystem from yaml (#681)

## Summary

dependabot is now suggesting all possible package upgrades, when we
really only want automated security updates: we'll decide when to update
versions via a separate process not just based on "availability".

## Details

It appears there's no entirely clear/reliable way to do that except from
the GitHub advanced security UI, and it appears that defining the
`github-actions` ecosystem behavior in the yaml file won't affect the
existing UI configuration for other ecosystems; so try just removing the
`uv` ecosystem configuration...

## Test Plan

Merge and wait ...

## Related Issues


---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: dbutenhof

.github/dependabot.yml

@@ -10,13 +10,3 @@ updates:
     labels:
       - "dependencies"
       - "build"
-  - package-ecosystem: "uv"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    open-pull-requests-limit: 10
-    commit-message:
-      prefix: "[UV]"
-    labels:
-      - "dependencies"
-      - "python:uv"