routing: let BlindedPaymentPathSet handle FinalCLTV logic

Instead of needing to remember how to handle the FinalCLTV value of a
blinded payment path at various points in the code base, we hide the
logic behind a unified FinalCLTVDelta method on the blinded path.

routing/pathfind.go

@@ -210,21 +210,12 @@ func newRoute(sourceVertex route.Vertex,
 			// reporting through RPC. Set to zero for the final hop.
 			fee = 0
 
-			// Only include the final hop CLTV delta in the total
-			// time lock value if this is not a route to a blinded
-			// path. For blinded paths, the total time-lock from the
-			// whole path will be deduced from the introduction
-			// node's CLTV delta. The exception is for the case
-			// where the final hop is the blinded path introduction
-			// node.
-			if blindedPathSet == nil ||
-				len(blindedPathSet.GetPath().BlindedPath.
-					BlindedHops) == 1 {
-
-				// As this is the last hop, we'll use the
-				// specified final CLTV delta value instead of
-				// the value from the last link in the route.
+			if blindedPathSet == nil {
 				totalTimeLock += uint32(finalHop.cltvDelta)
+			} else {
+				totalTimeLock += uint32(
+					blindedPathSet.FinalCLTVDelta(),
+				)
 			}
 			outgoingTimeLock = totalTimeLock