ipv6: Remove route peer binding assertions.

They are bogus.  The basic idea is that I wanted to make sure
that prefixed routes never bind to peers.

The test I used was whether RTF_CACHE was set.

But first of all, the RTF_CACHE flag is set at different spots
depending upon which ip6_rt_copy() caller you're talking about.

I've validated all of the code paths, and even in the future
where we bind peers more aggressively (for route metric COW'ing)
we never bind to prefix'd routes, only fully specified ones.
This even applies when addrconf or icmp6 routes are allocated.

Signed-off-by: David S. Miller <davem@davemloft.net>