mm: zswap: delete unused acomp->is_sleepable

mm: zswap: delete unused acomp->is_sleepable

This hasn't been used since 7d4c9629b74f ("mm: zswap: use object
read/write APIs instead of object mapping APIs").  Drop it.

Link: https://lkml.kernel.org/r/20251211025645.820517-1-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>