codeintel: Only shard symbols by repo

Created by: chrismwendt

Now that symbols does incremental indexing #27932, it's better to route all search requests for a given repo to a specific symbols service pod (to maximize incrementality and reuse of old indexes), rather than routing by repo@commit.

Suggested by @keegancsmith in https://github.com/sourcegraph/sourcegraph/issues/27611#issuecomment-971656158