ci: Remove greetings workflow

It is quite annoying, not very useful and takes 30–50 seconds to finish

.github/workflows/greetings.yml

@@ -1,15 +0,0 @@
-name: Greetings
-
-on: [pull_request, issues]
-
-jobs:
-  greeting:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/first-interaction@v1
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          issue-message: |
-            Hello! We're very happy to see your first issue. If your issue is about a problem, go back and check you have copy-pasted all the debug logs you can so we can help you as fast as possible!
-          pr-message: |
-            Hello! Thank you about this PR. Since this is your first PR, please make sure you have described the improvements and your code is well documented.