Cleanup Pleroma.Upload proxy opts

#2379 (closed) !3219 (merged)

The proxy_opts for Pleroma.Upload should not be configurable:

config :pleroma, Pleroma.Upload,
   filters: [Pleroma.Upload.Filter.Dedupe],
   link_name: false,
   proxy_remote: false,
   proxy_opts: [
     redirect_on_failure: false,
     max_body_length: 25 * 1_048_576,
     http: [
       follow_redirect: true,
       pool: :upload
     ]
   ],
   filename_display_max_length: 30,
   default_description: nil

These options are only used by the backend during retrieval of files from S3 storage. They do not do anything to affect the upload transaction.

redirect_on_failure: Wouldn't work if the S3 storage is not publicly accessible. Users can directly access it by whitelisting the domain from MediaProxy.

max_body_length: If this value is smaller than :instance, upload_limit it would prevent you from retrieving files you successfully uploaded

follow_redirect: makes sense to just leave this enabled just in case

pool: Only affects the retrieving of uploads. We don't apply a connection pool dedicated to the uploading of attachments at this time.