heal: Remove empty directories (#11354)

Since the introduction of __XLDIR__, an empty directory does not have a
meaning anymore in erasure mode. Make healing removes it wherever it
finds it.

cmd/storage-datatypes.go

@@ -76,6 +76,8 @@ type FileInfoVersions struct {
 	// Name of the file.
 	Name string
 
+	IsEmptyDir bool
+
 	// Represents the latest mod time of the
 	// latest version.
 	LatestModTime time.Time