Remove HU-only invoice processing, and add s3 upload function to specifically upload HU-BU invoice

[QuanMPhm]

Based on @joachimweyl's comments here and here, we no longer need to export a HU-only invoice, and will need to add a function to specifically export the HU-BU invoice. A dedicated function is needed because the HU-BU invoice filename's pattern is different from all other invoices being uploaded to S3, due to the MGHPCC's request.