Hash partitioning was never fully implemented, and was never turned
on by default. There has been no effort to complete the feature, so
rather than carrying dead code this removes all support for hash
partitioning. Should we ever want this feature, we will most likely
start from scratch anyways.

As an effect from removing the unsupported MERGE/MODIFY commands,
this previously accepted query is no longer legal:

	create table t (a int, b int)
	distributed by (a)
	partition by range (b) (start () end(2));

The syntax was an effect of an incorrect rule in the parser which
made the start boundary optional for CREATE TABLE when it was only
intended for MODIFY PARTITION.

pg_upgrade was already checking for hash partitions so no new check
was required (upgrade would've been impossible anyways due to hash
algorithm change).