Turn off default up/down scrolling from ScrollView's core:move-up/down event handling

Fixes #152.

When https://github.com/atom/atom/pull/2368 was merged, ScrollView gained the ability to scroll when the up and down arrow keys are pressed (more precisely, when `core:move-up` or `core:move-down` events are triggered).

Since Tree View uses a ScrollView, this means that `core:move-up` and `core:move-down` events both select the previous/next item in the Tree View, and make the Tree View scroll up/down.

This change turns off the scrolling behavior by removing the ScrollView's default event handler for the `core:move-up` and `core:move-down` events.

Of course, the alternative is to roll back the changes from https://github.com/atom/atom/pull/2368 (wondering if some other packages might have been affected?). Personally, I think that ScrollView _should_ support `core:move-up` and `core:move-down` for scrolling by default, since it's a *Scroll*View. If subclasses want to disable this behavior, they're free to do so (and it's simple). Still, would love a :+1: on this reasoning.

cc @probablycorey