We don't use ubf_select after r65495 / 5de7b3b4
("thread_pthread.c (native_ppoll_sleep): new eventfd (or pipe) for ubf"),
so we don't need to unregister the thread from the ubf list.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66382 b2dd03c8-39d4-4d8f-98ff-823fe69b080e