sstable_directory: process_sstable_dir: use directory_lister
sstable_directory: process_sstable_dir: use directory_lister

Simplify the implementation by using directory_lister get()
rather than lister::scan_dir.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla