arm64: numa: Remove the unused parent_node() macro

Commit a7be6e5a ("mm: drop useless local parameters of
__register_one_node()") removes the last user of parent_node().

The parent_node() macro in ARM64 platform is unnecessary.

Remove it for cleanup.

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>