simplify sort.Sort by using sort.Slice (#17066)

cmd/object-api-datatypes.go

@@ -552,13 +552,6 @@ type CompletePart struct {
 	ChecksumSHA256 string
 }
 
-// CompletedParts - is a collection satisfying sort.Interface.
-type CompletedParts []CompletePart
-
-func (a CompletedParts) Len() int           { return len(a) }
-func (a CompletedParts) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
-func (a CompletedParts) Less(i, j int) bool { return a[i].PartNumber < a[j].PartNumber }
-
 // CompleteMultipartUpload - represents list of parts which are completed, this is sent by the
 // client during CompleteMultipartUpload request.
 type CompleteMultipartUpload struct {