Attempting to un-indent multiple list items only un-indents the first item

Attempting to un-indent multiple list items in a bulleted or numbered list only un-indents the first item.
Related: #907

##### Steps to Reproduce

1. Create a list with multiple items.
2. Select all the items.
3. Press the un-indent button.

Here's a video demonstrating the issue:

https://user-images.githubusercontent.com/11605395/120842324-6724f500-c521-11eb-85f8-20d803fda034.mov

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

**Before**
```

• 
  
  Item 1
  


• 
  
  Item 2
  

```

**After (Result)**
```

• 
  
  Item 2
  

```

**Expected result**
```

```
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)