fix(transfer): dont create labels if missing

[weihanglo]

This is quite annoying if the target repo doesn't need those labels, or have a completely different label management approach.

While issues need to relabel anyway after transfer, it requires more steps to delete a label. I don't think it is worth.

Alternatively, we could have an argument @rustbot transfer cargo createMissingLabels=1 in the future, if people really want missing labels created.

[weihanglo]

And according to the triagebot doc:

WARNING: Transferring is a partially destructive command. For example, labels and milestones that don’t exist in the target repository will be removed from the issue.

[weihanglo]

Another alternative: configuring via the [transfer] table.

[transfer]
create-labels-if-missing = true

[apiraino]

This was introduced in #1780 and I don't see a specific discussion about labels, therefore I think it's ok to experiment with not migrating labels. I guess the reasoning is that labelling is very specific to the git repository and the workflow of maintainers working there, so there is very little overlap between codebases about labels.

[ehuss]

Thanks! I don't remember what happened here. That was not the behavior I intended. Perhaps I was testing things and forgot to change it back?