download: remove compatibility with old cache naming scheme

download: remove compatibility with old cache naming scheme

A long time ago, the name of cached files was derived from the source URL.
This was changed in 2011 with d46db43e21 ("Don't include the source URI in
the cached filename.")

Some compatibility code was left behind: even today, we are still trying
to read from the old filename.  The goal of this compatibility code was to
account for existing caches that still had files with the old naming
scheme.

More than 9 years later, it is safe to remove this compatibility code.
It simplifies the download code and avoids a useless disk access.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>