Revert "Revert "Reset primary_server when remove_server"" [ci skip]

This reverts commit d2671c96f32e6fbbd39162a2c9042dcaf3bf2d68.

previous reverting is commit miss

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65246 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/drb/drb.rb

@@ -1866,6 +1866,11 @@ module DRb
   # Removes +server+ from the list of registered servers.
   def remove_server(server)
     @server.delete(server.uri)
+    mutex.synchronize do
+      if @primary_server == server
+        @primary_server = nil
+      end
+    end
   end
   module_function :remove_server