arm64: mm: Remove pte_unmap_nested()

As of commit ece0e2b6 ("mm: remove pte_*map_nested()"),
pte_unmap_nested() is no longer used and can be removed from the arm64
code.

Signed-off-by: Qian Cai <cai@lca.pw>
[will: also remove pte_offset_map_nested()]
Signed-off-by: Will Deacon <will.deacon@arm.com>