One bus: the AMQP transport is gone from the host

The mesh runs on the seat's bus alone (novox/hq ADR 0131, design 28 task 5.5). The host's old
dialling and enrolment paths are deleted with the switch that chose between them; a membership or
a token naming another bus is refused before anything is sent, rather than dialled on a transport
that no longer exists.

internal/link/asking.go

@@ -2,6 +2,7 @@ package link
 
 import (
 	"context"
+	"fmt"
 	"time"
 )
 
@@ -16,9 +17,8 @@ import (
 // Approach is how a node reaches a mesh it does not yet belong to.
 //
 // The three things a token carries about where to go, and nothing about who is asking: the address,
-// the certificate that address must present, and which bus is at the other end. **Every token names
-// the bus the mesh runs on today until the rollout** (novox/hq ADR 0116 step 5), so an empty
-// Transport is the ordinary case rather than something missing.
+// the certificate that address must present, and which bus is at the other end — the mesh's own
+// (hearing.go), and a token naming any other is refused before anything is sent.
 type Approach struct {
 	Address     string
 	Fingerprint string
@@ -46,10 +46,8 @@ type Asking interface {
 func Present(ctx context.Context, to Approach, node, secret string,
 	timeout time.Duration) (Asking, error) {
 
-	switch to.Transport {
-	case OnNATS:
-		return presentNats(ctx, to, node, secret, timeout)
-	default:
-		return presentCurrent(ctx, to, node, secret, timeout)
+	if to.Transport != OnNATS {
+		return nil, fmt.Errorf("this token is for the %q bus, and the mesh's bus is %s", to.Transport, OnNATS)
 	}
+	return presentNats(ctx, to, node, secret, timeout)
 }