Reverts 2eb3841e9c
because it fails on "update-deps" check in the ruby/ruby CI.
https://github.com/ruby/ruby/runs/4230891140?check_suite_focus=true
```
diff --git a/ext/io/wait/depend b/ext/io/wait/depend
index 7b314b9..449e9fe 100644
--- a/ext/io/wait/depend
+++ b/ext/io/wait/depend
...
```
Maybe now it does not work on Ruby 2.6. This file must be changed for
each Ruby version. I have no good idea to fix this issue.