Remove snippet search

Proposal

Since snippet search is no longer being actively supported, often breaks and is hard to discover, we should remove snippet search as a clean up and to remove unnecessary data from Elasticsearch.

• Add a deprecation notice for removing snippet search before %17.7: Announce snippet search deprecation (#480315)
• Remove snippet search from SearchService and other places
• Remove snippet API search
• Stop indexing snippets
• Add migration to remove snippets from the main index
• Remove snippet proxy classes