Bug 1047283 - [Calendar] Drawer /Add event/ Display event should not play scrolling animation

the comment also seems redundant since the method is named `_shouldScroll` we already know there is a special condition that should be matched... if the `_shouldScroll` implementation is clear enough we don't need to explain what it does.

maybe we should explain that we don't want it to scroll when coming back from add/edit event screens. (that way the comment explains the "why" and not the "how")