Remove conditional assignment

HTML nodes are guaranteed to be detached before being re-attached, so we
are always sure that `subscriptions` is null on attach and non-null on
detach.

src/pane-axis-element.coffee

@@ -3,7 +3,7 @@ PaneResizeHandleElement = require './pane-resize-handle-element'
 
 class PaneAxisElement extends HTMLElement
   attachedCallback: ->
-    @subscriptions ?= @subscribeToModel()
+    @subscriptions = @subscribeToModel()
     @childAdded({child, index}) for child, index in @model.getChildren()
 
     switch @model.getOrientation()