Revert "Fix Socket.tcp cleanup after Thread#kill (ruby#15131)" (ruby#15565)

This reverts commit 3038286.

The following CI failure scared me:
https://github.com/ruby/ruby/actions/runs/20241051861/job/58108997049

```
  1) Timeout:
  TestResolvDNS#test_multiple_servers_with_timeout_and_truncated_tcp_fallback
```

Since it could be related, I'm reverting this for now.

Author: luke-gruber

ext/socket/lib/socket.rb

@@ -917,8 +917,7 @@ def self.tcp_with_fast_fallback(host, port, local_host = nil, local_port = nil,
     end
   ensure
     hostname_resolution_threads.each do |thread|
-      thread.kill
-      thread.join
+      thread.exit
     end
 
     hostname_resolution_result&.close