Remove KSQO from GUC and move file to _deadcode.

doc/src/sgml/runtime.sgml

src/backend/optimizer/plan/planner.c

src/backend/optimizer/prep/Makefile

src/backend/optimizer/prep/_deadcode/prepkeyset.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/bin/psql/tab-complete.c

src/include/optimizer/planmain.h