[ruby/json] remove redundant self.

ruby/json@c060943d04

Author: GrantBirki

ext/json/lib/json/common.rb

@@ -172,7 +172,7 @@ def generator=(generator) # :nodoc:
         end
       end
       self.state = generator::State
-      const_set :State, self.state
+      const_set :State, state
     ensure
       $VERBOSE = old
     end