mm/page_alloc: remove unnecessary __maybe_unused in order_to_pindex()

The `movable` variable is always used when `CONFIG_TRANSPARENT_HUGEPAGE`
is enabled, so the `__maybe_unused` attribute is not necessary.  This
patch removes it and keeps the variable declaration within the `#ifdef`
block for better clarity.

Link: https://lkml.kernel.org/r/20250319091726.401158-1-liuyerd@163.com


Signed-off-by: Liu <Ye&lt;liuye@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>