Make repository id optional in OpenAPI spec (#621)

The 'id' field in the Repository schema is now optional in both
openapi.yaml and server.schema.json for consistency.

Author: domdomegg

docs/reference/api/openapi.yaml

@@ -252,7 +252,6 @@ components:
       required:
         - url
         - source
-        - id
       properties:
         url:
           type: string