Do not check entities files for XML validity

[haszi]

Do not check entities files for XML validity as these are never valid on their own.

[haszi]

Test failures are unrelated to this PR.

[alfsb]

Pure XML Entities entities.* files should not exist on doc repositories since the idempotent build project. If there are still some, these need to be deleted. I found only two on doc-pl/reference/dir.

So no, for this change.

But as you are at it, changing broken.php to ignore any paths that contain reference/wkhtmltox/wkhtmltox/bits/ would make the warnings a little better. I was hopeful to move these these bits/ files as entities, so another exception on broken.php would not be necessary, but never got to do it.