Remove HASH_DIRSIZE, always use the default algorithm to select it

Remove HASH_DIRSIZE, always use the default algorithm to select it

It's not very useful to specify a non-standard directory size. The
HASH_DIRSIZE option was only used for shared memory hash tables, and
those always used hash_select_dirsize() to choose the size, which in
turn just uses the default algorithm anyway. That assumption was
ingrained in hash_estimate_size(), too.

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