Reduce `upload_files_parallel` API calls

[szokeasaurusrex]

In upload_files_parallel, we make an API call at the beginning of the method to obtain a list of existing files, then if we find we have a new file to upload, which would replace the existing one, we delete the existing file with a separate API call, and lastly, we upload the new file, also with a separate API call.

It appears this logic is designed to allow us to overwrite files that already exist on the server. We should instead perform this logic on the server, and remove this logic from the CLI. Having the logic on the CLI makes the system prone to race conditions, and slows uploads by adding unnecessary API calls.

[github-actions[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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