[TECH DEBT] cleanup old nightly repo files

[ITJamie]

https://repo.saltproject.io/salt-dev/salt/ and https://repo.saltproject.io/salt-dev/onedir/ are old build paths of salt onedir nightlys.

now that the new structure exists I would propose that we remove or empty these folders as they have caused confusion (eg salt-bootstrap is using the old URLs until saltstack/salt-bootstrap#1951 is merged)