Delete checksum after file deletion

[Poitrin]

WHY are these changes introduced?

Fixes Shopify/cli#733

Files that get added and uploaded, then deleted (both local and remote), then added again, will not get uploaded again, because checksums are outdated.

WHAT is this pull request doing?

When a file has been successfully deleted (both local and remote), checksum in checksum store is removed, so that the same file can be reuploaded again.

How to test your changes?

1. In a theme directory, run shopify theme serve
2. Add a new file, e.g. index.js with content console.log('…') in the assets folder, and verify that it gets uploaded.
3. Delete the file locally, and verify that it gets deleted remotely too.
4. Revert the delete, and verify that it gets uploaded again.

Post-release steps

• New CLI 3.x release

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above (if needed).

[karreiro]

Thank you, @Poitrin!