Drop support for GitHub

.github/PULL_REQUEST_TEMPLATE.md

-<!-- AlekSIS is developed on EduGit. GitHub only serves as
-     backup mirror and to help people find the project. If
-	 possible, please submit your merge request on EduGit!
-	 
-	 EduGit accepts logins with GitHub accounts.
--->
-
-[ ] I have read the above and have no way to contribute on EduGit
-[ ] I understand that GitHub's terms of service exclude young and 
-    learning contributors, but still cannot contribute on EduGit
-    instead.