realm/playground: Add API endpoint for deleting a playground entry.

Similar to the previous commit, we have added a `do_*` function
which does the deletion from the DB. The next commit handles sending
the events when both adding and deleting a playground entry.

Added the openAPI format data to zulip.yaml for DELETE
/realm/playgrounds/{playground_id}. Also added python and curl
examples to remove-playground.md.

Tests added.

zerver/lib/actions.py

@@ -6602,6 +6602,10 @@ def do_add_realm_playground(realm: Realm, **kwargs: Any) -> int:
     return realm_playground.id
 
 
+def do_remove_realm_playground(realm_playground: RealmPlayground) -> None:
+    realm_playground.delete()
+
+
 def get_occupied_streams(realm: Realm) -> QuerySet:
     # TODO: Make a generic stub for QuerySet
     """ Get streams with subscribers """