memory tier: use helper function destroy_memory_type()

Use helper function destroy_memory_type() to release memtype instead of
open code it to help improve code readability a bit.  No functional change
intended.

Link: https://lkml.kernel.org/r/20230626121053.1916447-1-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Wei Xu <weixugc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>