Motivation for this change

This PR updates the google-fonts package to a newer version.
There's some more post-processing to be done because of duplicate
fonts. The reasoning is documented and it generally seems to work.

I'm also removing the fixed output derivation hashes. Generally
they don't really add much in terms of reproducable builds. They
make updating the package more annoying and cumbersome. Plus, there's
the issue where if the build starts failing at some point, this will
be shadowed by the fact that people will just get hydra cached versions.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.