Revert "Support MSC3916 by adding a federation /download endpoint"

[anoadragon453]

Reverts #17172

This ended up causing an Exception when interacting with https://github.com/matrix-org/synapse-s3-storage-provider/

Click to view the traceback

  File "synapse/http/server.py", line 332, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "synapse/http/server.py", line 544, in _async_render
    callback_return = await raw_callback_return
  File "synapse/rest/media/thumbnail_resource.py", line 94, in on_GET
    await self.thumbnail_provider.respond_local_thumbnail(
  File "synapse/media/thumbnailer.py", line 282, in respond_local_thumbnail
    await self._select_and_respond_with_thumbnail(
  File "synapse/media/thumbnailer.py", line 512, in _select_and_respond_with_thumbnail
    responder = await self.media_storage.fetch_media(file_info)
  File "synapse/media/media_storage.py", line 232, in fetch_media
    res: Any = await provider.fetch(path, file_info, media_info, federation)
  File "synapse/logging/opentracing.py", line 921, in _wrapper
    return await func(*args, **kwargs)
  File "synapse/media/storage_provider.py", line 152, in fetch
    self.backend.fetch(path, file_info, media_info, federation)
TypeError: S3StorageProviderBackend.fetch() takes 3 positional arguments but 5 were given

Reverting for now to fix develop, with a fix coming later.

[anoadragon453]

Merging and then cherry-picking to matrix-org-hotfixes.