reposync: remove interrupted downloads

Yum does not seem to handle download resumes very well. In order to avoid errors like:

```
  Repo Sync Errors: (50, u'checksums did not match 326a904c2fbd7a0e20033c87fc84ebba6b24d937 vs afd8c60d7908b2b0e2d95ad0b333920aea9892eb', 'Invalid information uploaded  to the server')
```
Any remains in /var/spacewalk/packages/1/stage should get automatically deleted before spacewalk-repo-sync runs to have less errors when syncing patches.