[nova][ops] Removing Debug middleware

Deployers-

There's a Debug middleware in nova that's not used in the codebase since 2010, so we're removing it [1].

BUT

Theoretically, deployments could be making use of it by injecting it into the paste pipeline (a ~3LOC edit to your local api-paste.ini). If this is you, and you're really relying on this behavior, let us know. We can either revert the change or show you how to carry it locally (which would be really easy to do).

Thanks, efried

[1] https://review.opendev.org/#/c/662506/