Clean up optional rules in grammar
Various rules for optional keywords contained unnecessary rules and
type declarations. Remove those, thus making the output a tiny bit
smaller.
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/
e9eed669-e32d-6919-fed4-
acc0daea857b%40enterprisedb.com