Fix Markdown preview not working in Edit Milestone page

### What does this MR do?

This MR removes the automatic Zen Mode URL update when a hash is present and makes Markdown preview work again in the Edit Milestone page. I think the intent was to make it possible to link to a full-screen edit with a URL (e.g. http://foo/bar/issues/1/edit#fullscreen), but I don't this has ever worked. Perhaps a future MR can support this.

### Why was this MR needed?

A JavaScript error would be seen in the Milestone Edit page:

```javascript
Uncaught error, unrecognized expression: $('.zennable input[type=checkbox]##md-preview-holder')
```

In the Milestone Edit page, apparently the use of the hash `#md-preview-holder` causes the Zen Mode JavaScript to attempt to parse the hash for the `fullscreen_` prefix and look up the checkbox based on a unknown ID.

### Are there points in the code the reviewer needs to double check?

If we want to keep this hash update, an alternative fix is to add an `if` check in the beginning of `checkboxFromLocationHash`:

```coffeescript
if (window.location.hash.indexOf('#' + ZenMode.fullscreen_prefix) == -1)
  return null
```

### What are the relevant issue numbers?

* Closes #1687
* Closes https://github.com/gitlabhq/gitlabhq/issues/9325

See merge request !711