gnrc/ipv6: `nib route`: hide off-link PLEs

[xnumad]

Contribution description

An off-link prefix with SLAAC (i.e. RA PIO: L flag = 0, A flag = 1):

• is listed in nib prefix. Listing it here only means it’s a "managed" prefix (for SLAAC lifetimes), no indication about whether it’s on-link.
• nib route ❌ Currently falsely displays it as on-link (e.g. 2001:db8::/32 dev #6) Should not list it, to reflect the actually used routing logic [1] https://github.com/RIOT-OS/RIOT/blob/0e7636a463894c177a2982fd08f7e02cef83aa89/sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.c#L653

This PR removes it from the nib route output.

[1] which is correct ("forward the packet to a default router" - https://datatracker.ietf.org/doc/html/rfc4861#section-6.3.4)

Testing procedure

Expand

sudo dist/tools/tapsetup/tapsetup

In any directory: Set file content of radvd-offl.conf to:

interface tapbr0 {
    AdvSendAdvert on;
    prefix 2001:db8::/32 {
        AdvOnLink off;
    };
};

Run sudo radvd --nodaemon --config=radvd-offl.conf

cd examples/gnrc_networking/
make && make term

In RIOT terminal: command nib route In command output, observe unexpected line 2001:db8::/32 dev #6.

Issues/PRs references

#20757 revealed this bug by adding a feature