x86/mm: Remove the arch-specific pgd_leaf() definition

PGD huge pages are not supported yet, let's use the generic definition
in <linux/pgtable.h>.

[ mingo: Cleaned up the changelog. ]

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Link: https://lore.kernel.org/r/20250331081327.256412-6-bhe@redhat.com