mm/damon/core: do not call ops.cleanup() when destroying targets

mm/damon/core: do not call ops.cleanup() when destroying targets

damon_operations.cleanup() is documented to be called for kdamond
termination, but also being called for targets destruction, which is done
for any damon_ctx destruction.  Nobody is using the callback for now,
though.  Remove the cleanup() call under the destruction.

Link: https://lkml.kernel.org/r/20250712195016.151108-9-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>