helix: remove grammar source (#170683)

not needed at runtime, bloats the package closure unnecessarily

pkgs/applications/editors/helix/default.nix

···
   nativeBuildInputs = [ makeWrapper ];
 
   postInstall = ''
+    # not needed at runtime
+    rm -r runtime/grammars/sources
+
     mkdir -p $out/lib
     cp -r runtime $out/lib
   '';