Remove stable sort usage when not needed (#5586)

Stable sort is needed when we are sorting based on two or more
distinct elements. When equal elements are indistinguishable,
such as with integers, or more generally, any data where the
entire element is the key like `PartNumber`, stability is not
an issue.

cmd/fs-v1-multipart.go

@@ -417,7 +417,7 @@ func (fs *FSObjects) ListObjectParts(bucket, object, uploadID string, partNumber
 	for partNumber, etag := range partsMap {
 		parts = append(parts, PartInfo{PartNumber: partNumber, ETag: etag})
 	}
-	sort.SliceStable(parts, func(i int, j int) bool {
+	sort.Slice(parts, func(i int, j int) bool {
 		return parts[i].PartNumber < parts[j].PartNumber
 	})
 	i := 0

cmd/gateway/azure/gateway-azure.go

@@ -806,7 +806,7 @@ func (a *azureObjects) ListObjectParts(bucket, object, uploadID string, partNumb
 	for _, part := range partsMap {
 		parts = append(parts, part)
 	}
-	sort.SliceStable(parts, func(i int, j int) bool {
+	sort.Slice(parts, func(i int, j int) bool {
 		return parts[i].PartNumber < parts[j].PartNumber
 	})
 	partsCount := 0