nix-generate-from-cpan: remove unkown license

Leave license empty when it is not provided. This prevents packages from
failing to build because the license is set to
`stdenv.lib.licenses.unkown`.

There will still be a warning about the unkown license.

maintainers/scripts/nix-generate-from-cpan.pl

@@ -164,7 +164,7 @@ Readonly::Hash my %LICENSE_MAP => (
 
     # License not provided in metadata.
     unknown => {
-        licenses => [qw( unknown )],
+        licenses => [],
         amb      => 1
     }
 );