pgstat: remove stats_temp_directory.

pgstat: remove stats_temp_directory.

With stats now being stored in shared memory, the GUC isn't needed
anymore. However, the pg_stat_tmp directory and PG_STAT_TMP_DIR define are
kept, as pg_stat_statements (and some out-of-core extensions) store data in
it.

Docs will be updated in a subsequent commit, together with the other pending
docs updates due to shared memory stats.

Author: Andres Freund 
Author: Kyotaro Horiguchi 
Reviewed-By: Kyotaro Horiguchi 
Discussion: https://postgr.es/m/20220330233550[email protected]
Discussion: https://postgr.es/m/20220303021600[email protected]