database: truncate_table_on_all_shards: drop outdated TODO comment

The comment was added in 83323e155e
Since then, table::seal_active_memtable was improved to guarantee
waiting on oustanding flushes on success (See d55a2ac762), so
we can remove this TODO comment (it also not covered by any issue
so nobody is planned to ever work on it).

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
(cherry picked from commit 93b827c185)

replica/database.cc

@@ -2593,9 +2593,6 @@ future<> database::truncate_table_on_all_shards(sharded<database>& sharded_db, s
     dblog.trace("{} {}.{} and views on all shards", should_flush ? "Flushing" : "Clearing", s->ks_name(), s->cf_name());
     auto flush_or_clear = [should_flush] (replica::table& cf) {
             if (should_flush && cf.can_flush()) {
-                // TODO:
-                // this is not really a guarantee at all that we've actually
-                // gotten all things to disk. Again, need queue-ish or something.
                 return cf.flush();
             } else {
                 return cf.clear();