Remove alpha from purchase tester icon to upload to testflight (#2705)

### Description
While uploading purchase tester to testflight, we were getting these
errors:
```
 [Application Loader Error Output]: ERROR: [ContentDelivery.Uploader] Asset validation failed (90396) Invalid Icon. The watch application 'PurchaseTester.app/Watch/PurchaseTesterWatchOS.app' contains an icon file 'Icon Image-AppIcon-watch-1024x1024@1x.png' with an alpha channel. Icons should not have an alpha channel.
[Application Loader Error Output]: ERROR: [ContentDelivery.Uploader] Asset validation failed (90717) Invalid App Store Icon. The App Store Icon in the asset catalog in 'PurchaseTester.app/Watch/PurchaseTesterWatchOS.app' can't be transparent nor contain an alpha channel.
```

This icon was added in #2701. After trying it manually, removing the
alpha channel seems to have worked in fixing the issue while uploading.
This PR just changes that icon with a version without an alpha channel.