Change the `NoteMetadata` constructor not to take the tag by default

[mmagician]

Also, maybe we should change the NoteMetadata constructor not to take the tag by default. This way, we'd just do:

let metadata = NoteMetadata::new(sender_account_id, NoteType::Public)
    .with_attachment(attachment);

And in cases when the tag is required, we'd do:

let metadata = NoteMetadata::new(sender_account_id, NoteType::Public)
    .with_tag(tag)
    .with_attachment(attachment);

Originally posted by @bobbinth in #2334 (comment)