mm/page_alloc.c: remove unneeded codes in !NUMA version of build_zonelists()

mm/page_alloc.c: remove unneeded codes in !NUMA version of build_zonelists()

When CONFIG_NUMA=n, MAX_NUMNODES is always 1 because Kconfig item
NODES_SHIFT depends on NUMA.  So in !NUMA version of build_zonelists(), no
need to bother with the two for loop because code execution won't enter
them ever.

Here, remove those unneeded codes in !NUMA version of build_zonelists().

[bhe@redhat.com: remove unused locals]
Link: https://lkml.kernel.org/r/ZgQL1WOf9K88nLpQ@MiWiFi-R3L-srv
Link: https://lkml.kernel.org/r/20240326061134.1055295-5-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>