Dont gossip channels we would prune

[TheBlueMatt]

During LDK startup we often get a flood of gossip for channels that haven't seen an update in two years. Peers really shouldn't be sending us this garbage, but currently many do. LDK won't after we finish a prune which runs after we've been up for a bit, but until then we'll happily forward the garbage we get from our peers, which maybe we shouldnt do.