Motivation for this change

Removing two ancient versions of syncthing that have been know to be broken / unsupported for a while.

Removal as announced by @peterhoeg [1].

https://groups.google.com/forum/#!msg/nix-devel/CR1eldka5SU/eO-xyV4UAgAJ

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.