[Bug] Delete only api mapping when sharing custom domain

## Issue

**Setup :** There are multiple chalice applications pointing to the same base URL and different `url_prefix`.

**Triggering the error :** Invoke `chalice delete` on one of the applications, it deletes all of the mappings shared by other chalice apps/lambdas and deletes the custom domain in API Gateway.

## Expected Behaviour

Delete the custom domain from gateway only if it is only API Mapping associated with the custom domain. Otherwise, delete only the API mapping associated with itself.

Start with the chalice delete command and trace the API Gateway custom-domain and API-mapping cleanup path. Reproduce the setup with multiple Chalice applications sharing a base URL, then verify that deleting one application removes only its API mapping and removes the custom domain only when no mappings remain.

Written by the indexing model from the issue text.