lnrpc+routing: remove k shortest path finding

[joostjager]

This PR is the follow up of #2497. After deprecating the num_routes parameter in lnd version 0.6, the parameter and all k-shortest related code can now be removed.

[halseth]

Impossible not to like :)

[wpaulino]

LGTM - only minor thing is that the RPC is still named QueryRoutes.

[joostjager]

LGTM - only minor thing is that the RPC is still named QueryRoutes.

I didn't think about that, but changing that would break all clients, even the ones that only request a single route already now. Not renaming it requires users that rely on multiple routes to notice this change, because their application may silently behave differently.

[C-Otto]

Could you add some information for those users who need more than a single route?

[joostjager]

@C-Otto you should be able to implement this client side by using the ignore parameters of queryroutes.

[C-Otto]

@joostjager Yes, I learned about the Router RPC calls that give the information I need. Attempt a payment, figure out why it fails, ignore the culprit node/edge, try again. A bit cumbersome, but it might work. See the discussion/progress in C-Otto/rebalance-lnd#51 and #3206 in case you're interested.