Use built-in Bevy nine slices instead of `bevy_nine_slice_ui`

[rparrett]

We use bevy_nine_slice_ui because Bevy didn't have this feature when the project was started.

If I recall correctly, we didn't use native slicing when Bevy 0.13 was released because it didn't support tinting of the slices, or maybe there was some showstopping bug.

I think it should be possible to remove this dependency now.