x86: remove unused iommu_nr_pages

Impact: cleanup, remove dead code

The last usage was removed by the patch set culminating in

| commit e3c449f526cebb8d287241c7e82faafd9709668b
| Author: Joerg Roedel <joerg.roedel@amd.com>
| Date:   Wed Oct 15 22:02:11 2008 -0700
|
|     x86, AMD IOMMU: convert driver to generic iommu_num_pages function

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>