Remove legacy multi-auth code (__all_credentials)

In PYTHON-1314 we removed multi-auth (removed db.authenticate/db.logout) but to cut scope we did not remove all of the legacy multi-auth internals from the client/pool/connection. The following cleanup is still possible:

• Remove MongoClient._all_credentials (possible replace it with MongoClient._credentials if need be).
• Pass the credentials to the Pool class at construction time (via PoolOptions) instead of passing __all_credentials around to every operation.
• Simplify SocketInfo.check_auth now that a single credential can be configured.