remove Ractor#close

close_incoming by antoher ractor means there is no other messages
will be sent to the ractor, so Ractor.receive will block forever,
and it should raise and stop.

close_outgoing by antoher ractor means, ... I don't have good idea
to use it. It can be a private method.

Ractor#close calls both, but it does not make sense to call
different purpose methods, so I remove it.

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