remove unused powernv code v3

From: Christoph Hellwig
Date: Tue Jun 25 2019 - 10:52:50 EST

Hi all,

the powerpc powernv port has a fairly large chunk of code that never
had any upstream user. We generally strive to not keep dead code
around, and this was affirmed at least years Maintainer summit.

Changes since v2:
- fix incorrect opal stub removals that crept in in a rebase

Changes since v1:
- reword some commit message