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.
(cherry picked from commit 9c11264530019e44a01a262ff1ebe20ff7b7390e)
Automatic commit of package [spacewalk-backend] release [2.2.7-1].
Automatic commit of package [spacewalk-java] release [2.2.19-1].