Don't delete files that are always uploaded.

As part of our deployment process we want to upload our asset manifest to S3.  To do so we have added `manifest.yml` to `config.always_upload`.

However, when using a combination of `config.manifest = true` and `config.existing_remote_files = 'delete'` this file would always deleted from S3, as `manifest.yml` is not seen as a local file.

A simple fix for this is to remove `always_upload_files` from the list of files to be deleted, which seems to make sense.