Un-indenting list item doesn't properly un-indent children

Un-indenting a parent item of a bulleted or numbered list doesn't properly un-indent children. The same issue occurs when selecting all list items and attempting to un-indent.

##### Steps to Reproduce

1. Create a bulleted or numbered list with two items.
2. Indent the second item.
3. Un-indent the first item. (Or select both items and attempt to un-indent)

Here's a video demonstrating the issue:

https://user-images.githubusercontent.com/11605395/120840062-94bc6f00-c51e-11eb-8100-0aab68cc86d1.mov

Here's the HTML before and after attempting to un-indent:

**Before**
```

• 
  
  Item 1
  
  
  
  • 
    
    Item 2
    
  
  
  
  

```

**After (Result)**
```

• 
  
  
  
  • 
    
    Item 2
    
  
  
  
  

```

**Expected result**
```

• 
  
  Item 2
  

```
This is what _should_ happen:

##### Details

* Trix version: 1.3.1
* Browser name and version: Chrome 91, Safari
* Operating system: macOS, Android, iOS

(This issue doesn't appear to be platform-specific)