pgsql: Remove hashagg_avoid_disk_plan GUC.

Remove hashagg_avoid_disk_plan GUC.

Note: This GUC was originally named enable_hashagg_disk when it appeared
in commit 1f39bce0, which added disk-based hash aggregation.  It was
subsequently renamed in commit 92c58fd9.

Author: Peter Geoghegan
Reviewed-By: Jeff Davis, Álvaro Herrera
Discussion: https://postgr.es/m/9d9d1e1252a52ea1bad84ea40dbebfd54e672a0f.camel%40j-davis.com
Backpatch: 13-, where disk-based hash aggregation was introduced.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bcbf9446a2983b6452c19cc50050456be262f7c5

Modified Files
--------------
doc/src/sgml/config.sgml              |  17 ----
src/backend/optimizer/path/costsize.c |   1 -
src/backend/optimizer/plan/planner.c  | 162 ++++++++++++----------------------
src/backend/utils/misc/guc.c          |  10 ---
src/include/optimizer/cost.h          |   1 -
5 files changed, 55 insertions(+), 136 deletions(-)