dissect: always invalidate secondary arch partitions if we found primary arch

Let's suppress the secondary arch data, since we never ever want to
mount it if we found the primary arch.

Previously we only suppressed in the Verity case, but there's little
reason to entertain the idea of a secondary arch in non-Verity
environments either, we are not going to use them, and should not do
decryption or anything like that.