Update workflow for new augur clades version

[jameshadfield]

This updates the workflow to use augur after augur PR https://github.com/nextstrain/augur/pull/728 has been merged. It removes two rules from the workflow.

In the situation where the ncov workflow is not updated but augur is, the pipeline will proceed without error however the 'emerging_lineage' branch label will be missing. (This is because the add_branch_labels.py script is looking for the key 'clade_annotation' which is no longer present.)

In the situation where the ncov workflow is updated but augur is not, rule.emerging_lineages will error due to unknown arguments. You can see this in the failing CI for this PR 😉

The script add_branch_labels.py is no longer used, but not removed here, as it contains logic to export spike mutations as branch labels which may be useful at some point. If we do use this, it would be better to produce an intermediate node-data JSON with a custom branch label to avoid modifying the auspice JSON after export.

Release checklist

If this pull request introduces backward incompatible changes, complete the following steps for a new release of the workflow:

• [ ] Determine the version number for the new release by incrementing the most recent release (e.g., "v2" from "v1").
• [ ] Update docs/src/reference/change_log.md in this pull request to document these changes and the new version number.
• [ ] After merging, create a new GitHub release with the new version number as the tag and release title.

If this pull request introduces new features, complete the following steps:

• [ ] Update docs/src/reference/change_log.md in this pull request to document these changes by the date they were added.