Remove the duplicated notification API spec in rest-catalog-open-api.yaml

[flyrain]

Looks like the notification rest api is still in file "rest-catalog-open-api.yaml", it should've been moved to the file "notifications-api.yaml" when we separated them.
cc @HonahX

[HonahX]

This shall be fixed via #936, which update and rename rest-catalog-open-api.yaml so that it is fully identical to the upstream one