Remove alpha channel from App Store icon

App Store Connect rejected the previous build with the following error:

```
[Transporter Error Output]: ERROR ITMS-90717: Invalid App Store Icon. The App Store Icon in the asset catalog in 'WordPress.app' can't be transparent nor contain an alpha channel.
```

@mattmiklic kindly provided a new version with no alpha, which I
verified with:

```
sips -g hasAlpha /path/to/new-icon.png
```

Author: mokagio