Remove "related" searches

Poorly documented and never used anywhere properly.

- No core search backend supports it.
- MWSearch implements it but we've never turned it on.
- SolrStore copies MWSearch but it doesn't even work.
- CirrusSearch doesn't bother because it's silly.

Change-Id: I8b052e8e772030fba6c361fdb2775b10a7f7a15a
Properly deprecate srprop=score|hasrelated

Logs stats on usage, output param descriptions for callers.

Change-Id: I000b91ff4d81dc1563231cd3ae0b56952b9574fd
Merge "Remove "related" searches"
Merge "Properly deprecate srprop=score|hasrelated"