Remove greeter action - it broke and wasn't that useful anyway.

.github/workflows/greetings.yml

@@ -1,17 +0,0 @@
-name: Greetings
-
-on:
-  issues:
-    types: [opened]
-  pull_request_target:
-    types: [opened]
-
-jobs:
-  greeting:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/first-interaction@v1
-      with:
-        repo-token: ${{ secrets.GITHUB_TOKEN }}
-        issue-message: "Hi! Thanks for helping make Wesnoth even better!"
-        pr-message: "Hi! :D Since this is your first pull request, don't forget to add yourself to the [credits](https://github.com/wesnoth/wesnoth/blob/master/data/core/about.cfg) under Miscellaneous Contributors."