Don’t add item in Workspace.open if activateItem is false

We thought it was a bug that activateItem: false caused the
item not to be added, but it turned out there were package
tests that depended on this behavior.

Ideally, we should have an addItem option that exhibits this
behavior instead.

Signed-off-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

src/workspace.js

@@ -712,7 +712,6 @@ module.exports = class Workspace extends Model {
       pane.clearPendingItem()
     }
 
-    pane.addItem(item, options)
     this.itemOpened(item)
 
     if (options.activateItem !== false) {