vfio/iommu_type1: Remove unused pinned_page_dirty_scope in vfio_iommu

pinned_page_dirty_scope is optimized out by commit 01032156
("vfio/iommu_type1: Mantain a counter for non_pinned_groups"),
but appears again due to some issues during merging branches.
We can safely remove it here.

Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Message-Id: <20210412024415.30676-1-zhukeqian1@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>