During swapoff, try_to_unuse() makes sure that zswap_invalidate() is
called for all swap entries before zswap_swapoff() is called.  This means
that all zswap entries should already be removed from the tree.  Simplify
zswap_swapoff() by removing the trees cleanup code, and leave an assertion
in its place.

Link: https://lkml.kernel.org/r/20240124045113.415378-3-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>