pgsql: Remove operator_precedence_warning.

Remove operator_precedence_warning.

This GUC was always intended as a temporary solution to help with
finding 9.4-to-9.5 migration issues. Now that all pre-9.5 branches
are out of support, and 9.5 will be too before v14 is released,
it seems like it's okay to drop it. Doing so allows removal of
several hundred lines of poorly-tested code in parse_expr.c,
which have been a fertile source of bugs when people did use this.

Discussion: https://postgr.es/m/2234320.1607117945@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a676386b58bf7cd2df81baa43eb1713d3a2ec055

Modified Files
--------------
doc/src/sgml/config.sgml | 23 --
doc/src/sgml/syntax.sgml | 6 +-
src/backend/nodes/outfuncs.c | 3 -
src/backend/parser/gram.y | 43 +--
src/backend/parser/parse_expr.c | 450 +-------------------------
src/backend/parser/parse_target.c | 5 -
src/backend/utils/misc/guc.c | 10 -
src/backend/utils/misc/postgresql.conf.sample | 1 -
src/include/nodes/parsenodes.h | 3 +-
src/include/parser/parse_expr.h | 1 -
10 files changed, 10 insertions(+), 535 deletions(-)