Deprecate RPC's that read or modify repo git config files

RPCs: SetConfig, WriteConfig, DeleteConfig, AddRemote, RemoveRemote, ListRemotes

Why:

• Gitaly should be responsible for repo git config files
• We should not have to "replicate" git config in Gitaly HA

We should settle on some minimal, static Git config that gitaly needs in repo config files and apply that idempotently. And ideally, there would be no config we have to apply at the repo level.