chromium: Remove URL in sources.nix and updater.

The updater is now splitted between a shellscript and a Nix expression
file which contains helpers and lookup functions to reconstruct all
information needed in order to fetch the source tarballs.

This means, that the sources.nix now doesn't contain URLs and only
versions and the corresponding SHA256 hashes. Of course, right now this
sounds like it's unnecessary, but we're going to fetch binaries soon so
it's a good idea to not unnecessarily clutter up sources.nix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>