Removing sticky TOC

[Stephanie Hobson]

Hi All,

In preparation for the upcoming redesign
<https://blog.mozilla.org/opendesign/future-mdn-focus-web-docs/> I'm
removing the code that cases the right column "in this article" to become
"sticky" when scrolling down the page.

It's difficult to have the old and new designs living side by side in the
code base so this change will help make that process a little easier.

If you're curious about the redesign, we're hoping to launch it to beta
testers
<https://developer.mozilla.org/en-US/docs/MDN/Contribute/Howto/Be_a_beta_tester>
next week and we'll make a blog post with screen shots at the same time.

Thanks for your understanding,
Stephanie.

MDN Front-End Dev.