network/ndisc: do nothing for existing routes if on-link flag is zero

network/ndisc: do nothing for existing routes if on-link flag is zero

This effectively reverts commit 155d7a2c049cf866a0bfde8de371f09dfb3b6f29.

From RFC 4861 section 6.3.4:
> Note, however, that a Prefix Information option with the on-link flag
> set to zero conveys no information concerning on-link determination and
> MUST NOT be interpreted to mean that addresses covered by the prefix
> are off-link.

So, we should not drop previously configured routes when receieved a RA
with Prefix Information option without on-link flag.

Closes #28435.