Move ltree parentsel() selectivity function into /contrib/ltree.

contrib/ltree/expected/ltree.out

contrib/ltree/ltree.sql.in

contrib/ltree/ltree_op.c

src/backend/utils/adt/geo_selfuncs.c

src/backend/utils/adt/selfuncs.c

src/include/catalog/catversion.h

src/include/catalog/pg_proc.h

src/include/utils/selfuncs.h