Optimizer cleanup.

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/nodes/freefuncs.c

src/backend/nodes/readfuncs.c

src/backend/optimizer/README

src/backend/optimizer/geqo/geqo_paths.c

src/backend/optimizer/path/indxpath.c

src/backend/optimizer/path/joinpath.c

src/backend/optimizer/path/joinutils.c

src/backend/optimizer/path/mergeutils.c

src/backend/optimizer/path/orindxpath.c

src/backend/optimizer/path/prune.c

src/backend/optimizer/plan/createplan.c

src/backend/optimizer/util/pathnode.c

src/include/nodes/relation.h