lnd: remove deprecated endpoints `sendpayment` and `sendtoroute`

The endpoints sendpayment and sendtoroute have been deprecated more than 3 years(#4147), and now should be safe to remove them, https://github.com/lightningnetwork/lnd/blob/e02fd39ce6f38fec635325bf6bb5138b92fdfcfe/lnrpc/lightning.proto#L265-L267

https://github.com/lightningnetwork/lnd/blob/e02fd39ce6f38fec635325bf6bb5138b92fdfcfe/lnrpc/lightning.proto#L284-L286

To properly remove them:

1. update the test framework by removing SendToRoute and replace all the old calls with the newer SendToRouteV2.
2. remove the endpoints, which includes rpcserver.go, lightning.proto and other unit test files.