Remove Delete button to prevent accidental data loss

As discussed in https://github.com/atom/tree-view/pull/176#issuecomment-47172505

![screen shot 2014-06-26 at 2 38 23 pm](https://cloud.githubusercontent.com/assets/38924/3398367/d131eb96-fd2e-11e3-92c5-88bab60dfc77.png)

cc @probablycorey @abhibeckert