[FFE][designate] Pool Manager Removal and refactor

Hi All,

I am granting a FFE for 3 patches:

https://review.opendev.org/#/c/678432/ - Refactored service layer https://review.opendev.org/#/c/665476/22 - Removed deprecated pool-manager implementation https://review.opendev.org/#/c/657289/ - Removed deprecated powerdns 3 driver

These patches remove a duplicate code paths that have been deprecated for an extended period of time, and reduce a lot of the complexity in our implementation.

Please contact me if there is any issues or concern.

Thanks,

Graham