`seastar::at_exit()` was marked deprecated recently. so let's use the recommended approach to perform cleanups.
---
it's a cleanup, hence no need to backport.
Closesscylladb/scylladb#23253
* github.com:scylladb/scylladb:
perf/perf_sstable: fix the indent
perf/perf_sstable: stop using at_exit()