Make LaunchCluster not close DebugClient it's returning

LaunchCluster currently closes the gRPC connection underpinning the DebugService
it's returning. This causes all further calls to that service to immediately return with
a CANCELED error. There's no reason why it should do this (probably a refactoring artifact),
so just remove the Close call. Fixes T881.

Test Plan: CTS is not run as part of automated test still because it takes too long.

Bug: T881

X-Origin-Diff: phab/D671
GitOrigin-RevId: 6bfa382cba6a15b146b2f24311507456b58cdf98
diff --git a/metropolis/test/launch/launch.go b/metropolis/test/launch/launch.go
index af0b808..5fcfe5e 100644
--- a/metropolis/test/launch/launch.go
+++ b/metropolis/test/launch/launch.go
@@ -529,7 +529,6 @@
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to dial debug service: %w", err)
 	}
-	defer conn.Close()
 	debug := apb.NewNodeDebugServiceClient(conn)
 
 	if opts.NumNodes == 2 {