Don't clear the auth when disconnecting, it can mess up queries on the n...

...ode that are unauthorized due to a failover.

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When we restart our mongos' or have a stepdown, we've seen in-flight operations fail for being unauthorized and not reconnect. After debugging in our staging environment (I could never reproduce this locally with a local replica set and mongos), it seems that `@auth` is empty when the `query_failed?` so if `reply.unauthorized? && auth.has_key?(database)` evaluates to false. I added some log statements, and the auth gets cleared due to a disconnect.

I don't think we need to clear auth credentials on a disconnect, this also avoids this issue.