rollback #3358

#3358 was a workaround for a CodeMirror (actually Chrome seems to be the one at fault) bug drawing scrollbars over text, when there isn't anything to scroll. The workaround was a compromise, always drawing the space for the scrollbars to prevent the overlay, but reducing the margin so it isn't quite so annoying in the ~99% of the time that there are no scrollbars.  It turns out (#3397, #3399) that the margins are not reliable, so the margin adjustment could cause the very same problem.

closes #3391, #3397, #3399

For users who need to hide the scrollbars while zooming text while we wait for a CodeMirror release, you can add the following to your custom.css:

```css
.CodeMirror-scroll, .CodeMirror-wrap .CodeMirror-scroll {
 overflow: scroll;
}

.CodeMirror-vscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-hscrollbar {
   visibility: hidden;
}
```

or just put CodeMirror master in your profile's static/components directory.