Remove the spider feature

In preparation for the re-work of the DOWNLOAD helper to support
multiple download methods and protocols, the spider feature used with
wget is removed for now until it is re-implemented on top of the new
download methods.

Signed-off-by: Maxime Petazzoni <maxime.petazzoni@bulix.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>