Use `enqueue_#{column}_background_job?` to process in foreground

Pull request #169 sets `process_#{column}_upload = true` when `remove_#{column}` is set, even if it is set to false.
In this case, the file will not be processed in a background process.

In this pull request, I use `enqueue_#{column}_background_job?` (which checks if the file is marked for removal) instead of `process_#{column}_upload` to check if it should be a background process or if carrierwave should handle the processing directly.