It wasn't ever necessary. Still allow configuring this path in the GUI in case of different paths to static files for scratch-blocks.

This reverts commit 321839ca.

When loading an asset, construct the URL not by using the default data
type of the type of asset being loaded, but the actual data type of the
skeleton asset passed in. This allows the asset requestor to override
the data format, for example when loading a JPEG-format backdrop.