remove rbtdb64

During the all-hands meeting it came up that none of us remember the original reason for having the separate "rbtdb64" database implementation, which is identical to rbtdb except for using a 64-bit serial number type.

As far as I know, the only way to activate rbtdb64 is to select it with the database option in a zone statement. I suspect it's very rarely, if ever, used in production, and I think we can remove it in 9.13.