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.

ractor.rb

@@ -164,14 +164,6 @@ class Ractor
     }
   end
 
-  # Closes both incoming and outgoing ports.
-  def close
-    close_incoming
-    close_outgoing
-
-    self
-  end
-
   # utility method
   def self.shareable? obj
     __builtin_cexpr! %q{