stream_edl: disable caching

stream_edl merely makes demux_edl act "special", which checks for the
stream type explicitly and then does something with its URL. If a cache
is added before the stream, it'll try to use the cache's URL (i.e. an
empty string), and will then obviously fail to parse the URL. While this
is slightly stupid, just disabling the entirely useless cache is the
most effective solution.

Fixes #1378.