Ditch caching of current_releases as fedfind does it for us
    
    fedfind caches collections data anyway, so this is effectively
    duplicating that. (Also the one-day expiry wasn't working
    because fedfind's collections cache had no expiry; I've moved
    the one-day expiry logic to the fedfind cache in 4.2.8).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>