catfile: Remove gitaly-session-id

gitaly-session-id is a unique session that gets injected into the gRPC context before calls, that we use as part of the cache key for the catfile process cache. This was originally to achieve separation with regards to each rails request with the worry that if two rails requests shared the same cache that one affect the other.

However, the catfile cache is not a in memory cache that stores data per se, but a process cache that prevents from having to spawn a new git process for every time a client needs access to a git object.

This means that there is no reason why different requests for the same repository cannot share the same catfile process cache.

Let's remove the session ID from the cache key behind a feature flag.

fixes: #6375 (closed)