Don't discard unit keys unless VUK is available

Previously libaacs discarded unit keys when e.g. config file had 1 unit
key and the disc had 2 or more. This is fine when we have VUK or MK+VID
available. However, when VUK is not available, discarding unit key means
that the disc can't be decrypted.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
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