use default beamPackages for nix build

default.nix

@@ -1,14 +1,6 @@
 { lib
-, beam
-, callPackage
-, writeShellScriptBin
-, writeText
-, yarn2nix
-, mix2nix
-, fetchFromGitLab
+, beamPackages
 , fetchFromGitHub
-, fetchgit
-, fetchurl
 , git
 , cmake
 , nixosTests
@@ -18,10 +10,7 @@
 }:
 
 let
-  beamPackages = beam.packages.erlangR25.extend (self: super: {
-    elixir = super.elixir_1_14;
-  });
-  inherit (beamPackages) mixRelease buildMix buildRebar3 fetchHex;
+  inherit (beamPackages) mixRelease buildMix;
 in
 mixRelease rec {
   pname = "mobilizon";