Remove the code that writes and reads apc_cache::default_prefix from APCu

The value of apc_cache::default_prefix has been introduced to use a more random value instead of an empty string, but the code using that value is not able to understand if that value has been already set, and then removed from APCu, or it was never set. This means that the code is not able to understand if the APCu keys used for the cache must be invalidated.

The code that writes and reads apc_cache::default_prefix from APCu needs to be removed.