Turn off default up/down scrolling from ScrollView

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.

Fixes #152.
Merge pull request #153 from atom/iz-fix-tree-view-scroll

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