Fix scrolling for outputs.

Fixes #4434

We still have a problem with “new view of output” cutting off the bottom bit, since the toolbar takes up a bit of room at the top of the panel and the height 100% output is pushed down.

packages/outputarea/style/index.css

@@ -20,8 +20,6 @@
 
 .jp-OutputArea {
   overflow-y: auto;
-  display: flex;
-  flex-direction: column;
 }