Revert "netfilter: nft_numgen: add map lookups for numgen random operations"

Laura found a better way to do this from userspace without requiring
kernel infrastructure, revert this.

Fixes: 978d8f90 ("netfilter: nft_numgen: add map lookups for numgen random operations")
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>