Uncategorize leaf should delete thought

## Steps to Reproduce

```
- a
  - b
```

Activate Uncategorize on `b`

## Current Behavior

Nothing happens. 

## Expected Behavior

`b` should be deleted. 

When a thought with no childen (a leaf) is uncategorized, it should simply be deleted. 

The alert should still say "Uncategorize".

## Testing

Add a reducer test.