Remove moderate column from node_schema()

We no longer have any moderation in core, and haven't since Drupal 5, but the moderate column is still in schema for new installs. We should remove it, and let contrib modules like modr8 hook_schema_alter() it back if they need to. No db_drop_field() since we don't want to lose data here and it's still in use in contrib.

Note this should have a tiny positive performance impact since won't need to maintain this index any more, which means slightly faster inserts.