Simplify download filename parsing

Once https://github.com/AlexsLemonade/scpca-portal/issues/1460 is complete, we will be able to get the download filename directly from the API, so we woill be able remove the `parse_download_file` function.

Start with the `parse_download_file` function mentioned in the issue and check issue #1460 for the API change that will provide the download filename directly. Once that upstream change is complete, remove the obsolete parsing path and verify downloads use the filename returned by the API.

Written by the indexing model from the issue text.