Remove min threshold in P-Sig

[joyceyuu]

I don't think that filtering rare signatures is a good idea. I though we get the privacy protection by inserting the signatures into a Bloom filter. By carefully restricting the size of the filter, you will get a certain amount of collisions, and thus reducing the effectiveness of frequency attacks.

[gusmith]

Instead of removing it, should the default one be 0? Or the value enabling not to filter?