Actions: Remove commit-validation.yml workflow.

This is useless because it runs after the push; we need this
check for pull requests but not for core committers. (They should
use a pre-commit hook.)

.github/workflows/commit-validation.yml

@@ -1,39 +0,0 @@
-name: commit-validation
-on: [ push ]
-
-permissions:
-  contents: read
-
-jobs:
-  check-commit-msg-length:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v6
-    - name: Check commit message length
-      id: check-commit-msg-length
-      uses: actions/github-script@v8
-      with:
-        result-encoding: json
-        script: |
-          var longlines = 0;
-          const commits = ${{ toJSON(github.event.commits) }};
-          for (const commit of commits) {
-            for (const line of commit.message.split('\n')) {
-              if (line.length > 78) {
-                 longlines += 1;
-                 console.log("Overlong line:\n" + line);
-              }
-            }
-          }
-          return longlines
-    - name: Get result
-      run: |
-        result=${{steps.check-commit-msg-length.outputs.result}}
-        if [[ $result -eq 0 ]]; then
-          echo "Ok"
-          exit 0
-        else
-          echo "Commit messages contain $result lines longer than 78 characters."
-          echo "See under 'Check commit message length' for a list of the lines."
-          exit 1
-        fi