github: delete unneeded close_prs workflow

Who could have predicted that github would actually add a button to
disable PRs after so many years...

 .github/workflows/close_prs.yml | 21 ---------------------
 1 file changed, 21 deletions(-)

diff --git a/.github/workflows/close_prs.yml b/.github/workflows/close_prs.yml
deleted file mode 100644
index 805bdea..0000000
--- a/.github/workflows/close_prs.yml
+++ /dev/null
@@ -1,21 +0,0 @@
-# SPDX-FileCopyrightText: © 2024 The River Developers
-# SPDX-License-Identifier: 0BSD
-
-name: Close Pull Request
-
-on:
-  pull_request_target:
-    types: [opened]
-
-jobs:
-  run:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: superbrothers/close-pull-request@v3
-      with:
-        comment: |
-          Thanks for your interest in contributing to river!
-
-          Unfortunately, you are in the wrong place. As stated in the README, this github repo is a read-only mirror and development happens on codeberg: https://codeberg.org/river/river.
-
-          Please open a pull request on codeberg instead.