Critical: chalice delete is deleting the entire custom domain name in API Gateway

I've just run `chalice delete` to undeploy an api, and after that, chalice deleted my custom domain name from API Gateway entirely with all the mappings. It should delete only one mapping and not all of then. I've just lose the entire production stage mappings.

The bug happens when you have api_gateway_custom_domain configured for a domain in many other projects, running `chalice delete` for one of them will make you lose the custom domain name entirely.

That's a critical bug.

Begin at the `chalice delete` command and trace its API Gateway custom-domain cleanup behavior. Reproduce the case with one domain shared by multiple projects, then verify that deleting one project removes only its mapping and preserves the custom domain and other mappings.

Written by the indexing model from the issue text.