* s3 remote: honor s3.support_tagging in UpdateFileMetadata
The write path already skips tagging when the remote is configured
without tagging support, but the metadata-update path sent
PutObjectTagging or DeleteObjectTagging unconditionally. On remotes
that reject tagging requests, any metadata update failed -- including
updates with no tags at all, which land on the DeleteObjectTagging
branch.
* s3 remote: drop the never-read supportTagging field
Every maker set it, nothing read it: the tagging decision is made from
conf.S3SupportTagging. Keeping a field that looks like the switch but
is not invites exactly the inconsistency the previous commit fixed.