Automatically clean up migration archives stored on the Bitbucket Server instance

When migrating a Bitbucket Server repo with `gh bbs2gh migrate-repo`, we make an API request to the Bitbucket Server instance to generate a migration archive, and that migration archive is deposited onto the Bitbucket Server instance's disk.

We copy the file off using `scp`, but that file will remain on the Bitbucket Server instance taking up space. It is not clear from Atlassian's docs when, if ever, that archive will be purged.

I propose that, by default, we delete the archive from the Bitbucket Server instance once we have copied it off of the Bitbucket Server instance.

To give users additional control, we may want to allow this behaviour to be disabled, leaving the archive on disk.

Before deciding to work on this issue, we should consider if we want to offer some kind of resume/retry functionality where the migration fails on GitHub's size. If the migration archive was still available (i.e. not deleted), we could conceivably use it rather than starting a new export.