table: filter_sstable_for_reader: refactor clustering filtering conditional expression
table: filter_sstable_for_reader: refactor clustering filtering conditional expression

We're about to drop `filtering_broken` in a future patche
when clustering filtering can be supported for md-format sstables.

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

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