Remove partName, partETag requirement (#9044)

This is a precursor change before versioning,
removes/deprecates the requirement of remembering
partName and partETag which are not useful after
a multipart transaction has finished.

This PR reduces the overall size of the backend
JSON for large file uploads.

cmd/fs-v1-multipart.go

@@ -575,7 +575,6 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 
 		fsMeta.Parts[i] = ObjectPartInfo{
 			Number:     part.PartNumber,
-			ETag:       part.ETag,
 			Size:       fi.Size(),
 			ActualSize: actualSize,
 		}