DEV: extract leave_group method from the group#remove_member method (PR #13823)

Exactly the same fix as the fix with extracting the join method from the group#add_memebers method. See description of that fix - DEV: extract join_group method from groups#add_members method by AndrewPrigorshnev · Pull Request #13807 · discourse/discourse · GitHub.

GitHub

The title of this pull request changed from “DEV: extract the leave method from the group#remove_member method” to "DEV: extract leave_group method from the group#remove_member method