Deploying a webpacker project without sprockets fails

### Steps to reproduce
Deploy a project with assets built with Webpacker and without sprockets generating anything.

### Expected behavior
Deploy succeeds

### Actual behavior
Deploy fails because manifest files are missing
```
DEBUG [a53efae3] cannot access '/home/deploy/appname/releases/20190911225157/public/assets/.sprockets-manifest*'
DEBUG [a53efae3] : No such file or directory
DEBUG [769a050c] cannot access '/home/deploy/appname/releases/20190911225157/public/assets/manifest*.*'
DEBUG [769a050c] : No such file or directory
```

### Hot-Fix
Adding a line to **deploy.rb**
`Rake::Task["deploy:assets:backup_manifest"].clear_actions`

Start with deploy.rb and the deploy:assets:backup_manifest task, then reproduce a deployment using Webpacker without Sprockets. Check how missing public/assets manifest files are handled; done means the deployment succeeds without requiring Sprockets-generated manifests and the existing hot-fix is no longer needed.

Written by the indexing model from the issue text.