One bus: the AMQP transport is gone from the controller

The mesh runs on the seat's bus alone (novox/hq ADR 0131, design 28 task 5.5). The old
transport's consume loop, build request, tool ask, management API and account scoping are
deleted, and the bus switch with them; the controller connects to the broker seat and to
nothing else. The store-window tests keep their assertions on a bus-less fake, and the tests
that only made sense for the old transport's in-memory holding go with it.

internal/link/receive.go

@@ -78,19 +78,6 @@ type Control interface {
 	// Took settles the message: acted on, or understood and needing no action.
 	Took() error
 
-	// About names what this message is about — a node's report, one module's move, one build's
-	// outcome — and is said before the store is asked.
-	//
-	// A transport that holds messages **in memory** uses it to set aside anything older it is
-	// holding about the same thing: the older is the past, and letting it come back after the
-	// newer was acted on would undo the newer.
-	//
-	// **This is the one thing holding-in-memory can do that holding-in-the-server cannot**, and
-	// naming it here rather than hiding it is deliberate. On the bus being built the message
-	// belongs to the server and comes back whatever happened meanwhile, so this is ignored and the
-	// digest a report carries answers the same question instead (window.go, design 25 §3).
-	About(what string)
-
 	// Hold keeps the message and asks for it again after the delay — the store window.
 	Hold(after time.Duration) error