element-desktop: avoid duplicating element-web

https://github.com/NixOS/nixpkgs/pull/563892 added webapp.asar to
satisfy electron-builder. This unintentionally left it in the app bundle
(in addition to symlinked electron-web), bloating the package by ~140MB
for no good reason.

pkgs/by-name/el/element-desktop/package.nix

@@ -112,6 +112,9 @@ stdenv.mkDerivation (finalAttrs: {
 
     asar pack tmp-app "$packed"
 
+    # element-web is linked into the output during installPhase.
+    find ./dist -name webapp.asar -delete
+
     runHook postBuild
   '';