perl-packages.nix: cleanup meta

There are definetively a few Linux-only and Unix-only Perl packages, but `meta.platform` looks like it set at random, presumable by copy-pasting from code blocks nearby.
Many packages marked as Unix-only are in fact cross-platform and works well on Windows (and presumable on every platform where Perl runs)
So let's reset those per-package platform limitations, until we will have a reliable source of information for `meta.platform`.

Also removed empty `meta.maintainers` and `meta.license = "unknown"`