chore: s.remove_staging_dirs() should only be called once (#318)
chore: s.remove_staging_dirs() should only be called once (#318)

There is [an issue](https://github.com/googleapis/python-bigtable/blob/master/owlbot.py#L33) in the `owlbot.py` file added in #313 in that [s.remove_staging_dirs()](https://github.com/googleapis/synthtool/blob/master/synthtool/transforms.py#L309) should only be called once after all the files are copied over.  [get_staging_dirs()](https://github.com/googleapis/synthtool/blob/master/synthtool/transforms.py#L280) will only return staging directories that exist.
googleapis/python-bigtable,googleapis/python-bigtable