range_tombstone_change_generator: flush: use tri_compare rather than less
range_tombstone_change_generator: flush: use tri_compare rather than less

less is already using tri_compare internally,
and we'll use tri_compare for equality in the next patch.

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

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