ipv6ll: don't regenerate the address when it's removed externally

Currently if the IPv6 link-local address is removed after it passed DAD, NetworkManager tries to generate a new link-local address. If this fails, which is always the case for EUI64, ipv6ll is considered as failed and the connection can go down (depending on may-fail).

This is particularly bad for virtual interfaces because if somebody removes the link-local address, the activation can fail and destroy the interface, breaking all services that require it. Also, it's a change in behavior introduced in 1.36.0.

It seems that a better approach here is to just ignore the removal instead of generating a new address.

Fixes: aa070fb8 ('core: add NML3IPv6LL helper')