Remove affix plugin from issuable sidebar with new navigation
What does this MR do?
We don't need the affix plugin on the issuable sidebar when the new navigation is turned on. This causes a scroll event listener to be attached to the page but it doesn't actually do anything.
Looking at https://gitlab.com/gitlab-org/gitlab-ce/uploads/3f5158c4f9cd68f2702b8e72706b7b93/Screen_Shot_2017-08-09_at_19.20.34.png its not going to give us a massive performance benefit. But we will get a nice little gain that should be noticeable, especially with scrolling.