Revert "This spec actually depends on fchdir being available"

* This reverts commit 2d690c3.
* This change is problematic because if a Ruby implementation doesn't
  implement chdir it will then ignore this spec, but it should be a
  failure to not implement fchdir (on POSIX platforms).
* See discussion on 2d690c3

Author: eregon

core/dir/chdir_spec.rb

@@ -177,7 +177,7 @@ def to_str;  DirSpecs.mock_dir; end
       dir.close
     end
 
-    guard -> { Dir.respond_to? :fchdir } do
+    platform_is_not :windows do
       it "does not raise an Errno::ENOENT if the original directory no longer exists" do
         dir_name1 = tmp('testdir1')
         dir_name2 = tmp('testdir2')