nat: T6345: source NAT port mapping "fully-random" is superfluous in Kernel >=5.0

random - In kernel 5.0 and newer this is the same as fully-random. In earlier
kernels the port mapping will be randomized using a seeded MD5 hash mix using
source and destination address and destination port.

https://git.netfilter.org/nftables/commit/?id=fbe27464dee4588d906492749251454
(cherry picked from commit 7fe568ca1672f1dfbd2b56ee3ef7a6ab48b03070)