Remove HASH_SEGMENT option

Remove HASH_SEGMENT option

It's been unused forever. There's no urgency in removing it now, but
it was just something that caught my eye.

Aleksander Alekseev proposed this a long time ago [0], but Tom Lane
was worried about third-party extensions using it. I believe that's a
non-issue: I tried grepping through all extensions found on github and
didn't find any references to HASH_SEGMENT.

[0] https://www.postgresql.org/message-id/20160418180711.55ac82c0@fujitsu

Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://www.postgresql.org/message-id/01ab1d41-3eda-4705-8bbd-af898f5007f1@iki.fi