The `responses` property of `WebApps_Get` operation should not include `404`

Original issue: https://github.com/terraform-providers/terraform-provider-azurerm/issues/2704

Problematic swagger: https://github.com/Azure/azure-rest-api-specs/blob/09ac2b33d780dbf2f74de2083012f62aaa33c451/specification/web/resource-manager/Microsoft.Web/stable/2019-08-01/WebApps.json#L130

A `GET` operation is supposed to return explicit errors when the client received `404` which indicates the resource does not exist, and it would never be the result that the client is expecting.

Open the linked Microsoft.Web stable 2019-08-01 WebApps.json at the WebApps_Get operation around line 130 and inspect its responses definition. Confirm that the 404 response is listed there, then update the specification so the operation no longer includes it and run the repository's relevant specification validation. Done means the WebApps_Get responses match the stated GET error behavior.

Written by the indexing model from the issue text.