Revert "mm: make alloc_demote_folio externally invokable for migration"

Revert "mm: make alloc_demote_folio externally invokable for migration"

This reverts commit a00ce85af2a1be494d3b0c9457e8e81cdcce2a89.

Commit a00ce85af2a1 ("mm: make alloc_demote_folio externally invokable for
migration") was made to let DAMOS_MIGRATE_{HOT,COLD} call the function.
But a previous commit made DAMOS_MIGRATE_{HOT,COLD} call
alloc_migration_target() instead.  Hence there are no more callers of the
function outside of vmscan.c.  Revert the commit to make the function
static again.

Link: https://lkml.kernel.org/r/20250616172346.67659-4-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Honggyu Kim <honggyu.kim@sk.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>