Function remove_censored_exts_sharded 

pub fn remove_censored_exts_sharded<K: Kmer, D>(
    stranded: bool,
    valid_kmers: &mut [(K, (Exts, D))],
    all_kmers: &[K],
)

Remove extensions in valid_kmers that point to censored kmers. A censored kmer exists in all_kmers but not valid_kmers. Since the kmer exists in this partition, but was censored, we know that we can delete extensions to it. In sharded kmer processing, we will have extensions to kmers in other shards. We don’t know whether these are censored until later, so we retain these extension.