remove support for list bucketing "stored as directories" in 3.0

From the email thread:

1) LB, when stored as directories, adds a lot of low-level complexity to Hive tables that has to be accounted for in many places in the code where the files are written or modified - from FSOP to ACID/replication/export.
2) While working on some FSOP code I noticed that some of that logic is broken - e.g. the duplicate file removal from tasks, a pretty fundamental correctness feature in Hive, may be broken. LB also doesn’t appear to be compatible with e.g. regular bucketing.
3) The feature hasn’t seen development activity in a while; it also doesn’t appear to be used a lot.

Keeping with the theme of cleaning up “legacy” code for 3.0, I was proposing we remove it.

(2) also suggested that, if needed, it might be easier to implement similar functionality by adding some flexibility to partitions (which LB directories look like anyway); that would also keep the logic on a higher level of abstraction (split generation, partition pruning) as opposed to many low-level places like FSOP, etc.