powerpc/pci: Remove unimplemented prototypes

The corresponding definitions were deleted in commit 3d5134ee
("[POWERPC] Rewrite IO allocation & mapping on powerpc64") which
was merged a mere 13 years ago.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200902035138.1762531-1-oohall@gmail.com