beam-modules/hex-snapshot: remove

The package set is not maintained. It is also not used by most of the
BEAM community. Removing it to allow a more useful set of tools fit to
the BEAM community in Nixpkgs.

pkgs/development/beam-modules/build-mix.nix

@@ -1,4 +1,4 @@
-{ stdenv, writeText, elixir, erlang, hexRegistrySnapshot, hex, lib }:
+{ stdenv, writeText, elixir, erlang, hex, lib }:
 
 { name
 , version
@@ -43,7 +43,7 @@ let
     else setupHook;
 
     inherit buildInputs;
-    propagatedBuildInputs = [ hexRegistrySnapshot hex elixir ] ++ beamDeps;
+    propagatedBuildInputs = [ hex elixir ] ++ beamDeps;
 
     configurePhase = if configurePhase == null
     then ''