Apply more mechanical simplifications using recent Go features

This is a follow-up on the `go fix` run in commit
0210c5fd92.

That commit recorded the automated updates.

This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.

It also removes some now-obsolete comments and statements.

internal/smtpsrv/server.go

@@ -340,9 +340,7 @@ func (s *Server) ListenAndServe() {
 
 	// Never return. If the serve goroutines have problems, they will abort
 	// execution.
-	for {
-		time.Sleep(24 * time.Hour)
-	}
+	select {}
 }
 
 func (s *Server) serve(l net.Listener, mode SocketMode) {