do NOT immediately remove files from dev-downloads.libreoffice.org on update of externals!

[Bjoern Michaelsen]

Hi all,

please do NOT remove files from dev-downloads when updating externals, keep
those files around for at least 12 months! Otherwise you will break bibisect
building hard (apart from also breaking people on local branches).
So please keep whatever tarball was ever used in a LibreOffice build on the
server for at least 12 months.

Thanks,

Bjoern