remove ursa. resolves #1658.

Couldn't figure out the scrim, as the callback path for this particular operation is somewhat tortured:

https://github.com/xtuple/xtuple/blob/faacbfa3a7b8b7595592e91fb72620f6a4b32c65/enyo-client/extensions/source/oauth2/client/models/oauth2.js#L85-L91

So I just added a few sentences to the notify popup.
I think this is good advice: https://github.com/digitalbazaar/forge/issues/125#issuecomment-45674260

Even though slower than ursa, this will ensure it'll run as fast as possible on the diverse hardware this app will be running on. Cursory tests on my lapotp show that just increasing `workers` to 8+ yields significant performance improvements.