re-use transport for AdminInfo() call (#14571)

avoids creating new transport for each `isServerResolvable`
request, instead re-use the available global transport and do
not try to forcibly close connections to avoid TIME_WAIT
build upon large clusters.

Never use httpClient.CloseIdleConnections() since that can have
a drastic effect on existing connections on the transport pool.

Remove it everywhere.

cmd/storage-interface.go

@@ -63,7 +63,6 @@ type StorageAPI interface {
 	// returns 'nil' once healing is complete or if the disk
 	// has never been replaced.
 	Healing() *healingTracker
-
 	DiskInfo(ctx context.Context) (info DiskInfo, err error)
 	NSScanner(ctx context.Context, cache dataUsageCache, updates chan<- dataUsageEntry) (dataUsageCache, error)