[ruby/io-wait] Revert ruby/io-wait#9 "Make

the gem a noop on Rubies older than
2.6"

This reverts commit ruby/io-wait@75fcb74c327f.

The version that does nothing with pre-ruby 2.6 has already been
released, so there is no longer need to consider older rubies in newer
versions.

ruby/io-wait@930d2f0d07

Author: nobu

ext/io/wait/extconf.rb

@@ -1,25 +1,21 @@
 # frozen_string_literal: false
 require 'mkmf'
 
-if RUBY_VERSION < "2.6"
-  File.write("Makefile", dummy_makefile($srcdir).join(""))
+target = "io/wait"
+have_func("rb_io_wait", "ruby/io.h")
+have_func("rb_io_descriptor", "ruby/io.h")
+unless macro_defined?("DOSISH", "#include <ruby.h>")
+  have_header(ioctl_h = "sys/ioctl.h") or ioctl_h = nil
+  fionread = %w[sys/ioctl.h sys/filio.h sys/socket.h].find do |h|
+    have_macro("FIONREAD", [h, ioctl_h].compact)
+  end
+  if fionread
+    $defs << "-DFIONREAD_HEADER=\"<#{fionread}>\""
+    create_makefile(target)
+  end
 else
-  target = "io/wait"
-  have_func("rb_io_wait", "ruby/io.h")
-  have_func("rb_io_descriptor", "ruby/io.h")
-  unless macro_defined?("DOSISH", "#include <ruby.h>")
-    have_header(ioctl_h = "sys/ioctl.h") or ioctl_h = nil
-    fionread = %w[sys/ioctl.h sys/filio.h sys/socket.h].find do |h|
-      have_macro("FIONREAD", [h, ioctl_h].compact)
-    end
-    if fionread
-      $defs << "-DFIONREAD_HEADER=\"<#{fionread}>\""
-      create_makefile(target)
-    end
-  else
-    if have_func("rb_w32_ioctlsocket", "ruby.h")
-      have_func("rb_w32_is_socket", "ruby.h")
-      create_makefile(target)
-    end
+  if have_func("rb_w32_ioctlsocket", "ruby.h")
+    have_func("rb_w32_is_socket", "ruby.h")
+    create_makefile(target)
   end
 end