p2.mirrorsURL are kept in site metadata after mirroring with format attribute

When we mirror sites using the "format" argument (to keep the original repository layout with .pack.gz as siblings) it also keep the p2.mirrorsURL attribute. This is not bad per se, but since the mirrors are often not correctly sync'd with the Eclipse repositories/our mirrors, it leads to build failure.
Current workaround is to remove them by end in content.jar and artifact.jar.
We should think about removing this, for example as part of the remove-references script.