Update Privacy Calculations in Architecture Document

[dvorak42]

The privacy calculations should be updated to take into account metadata more explicitly, and take into account the cost of the number of redemptions, rather than the global issuer count.

If used in a multiple disjoint architecture ecosystem, then the math should also be taken in terms of a specific 'context' rather than treating the whole PrivacyPass ecosystem as a single global system (allowing for more nuanced requirements/limitations on issuer counts).

[chris-wood]

On reflection, we may be better off deleting the text entirely and making sure the justification for the parameterization is adequately covered in the security and privacy considerations elsewhere in the document.