Remove the periodically-scheduled Markdown link check. (#8580)

The state of links in our documentation is now sufficiently good that we are
running link checks during PRs. There is no longer any practical benefit to
running the scheduled "global" check. Most of the errors it reports are rate
limitations anyway (429).

.github/workflows/linkchecker.yml

@@ -1,12 +0,0 @@
-name: Check Markdown links
-on:
-  schedule:
-    - cron: '* */24 * * *'
-jobs:
-  markdown-link-check:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v3
-      - uses: creachadair/github-action-markdown-link-check@master
-        with:
-          folder-path: "docs"