Disable single-sstable tombstone compactions for DTCS

We should probably disable tombstone compactions by default for DTCS for these reasons:

1. users should not do deletes with DTCS
2. the only way we should get rid of data is by TTL - and then we don't want to trigger a single sstable compaction whenever an sstable is 20%+ expired, we want to drop the whole thing when it is fully expired