gcs: Fix upload errors when uploading pre 1970 files

Before this change rclone attempted to set the "updated" field in
uploaded objects to the modification time.

However when this modification time was before 1970, google drive
would return the rather cryptic error:

    googleapi: Error 400: Invalid value for UnsignedLong: -42000, invalid

However API docs: https://cloud.google.com/storage/docs/json_api/v1/objects#resource
state the "updated" field is read only and tests confirm that.  Even
though the field is read only, it looks like Google parses it.

This change therefore removes the attempt to set the "updated" field
(which was doing nothing anyway) and fixes the problem uploading pre
1970 files.

See #3196 and https://forum.rclone.org/t/invalid-value-for-unsignedlong-file-missing-date-modified/3466

backend/googlecloudstorage/googlecloudstorage.go

@@ -1003,7 +1003,6 @@ func (o *Object) Update(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOptio
 		Bucket:      o.fs.bucket,
 		Name:        o.fs.root + o.remote,
 		ContentType: fs.MimeType(src),
-		Updated:     modTime.Format(timeFormatOut), // Doesn't get set
 		Metadata:    metadataFromModTime(modTime),
 	}
 	var newObject *storage.Object