torch: don't use default value for src

pkgs/applications/science/machine-learning/torch/torch-distro.nix

···
 { luarocks, lib , stdenv,  writeText , readline,  makeWrapper,
   less, ncurses, cmake, openblas, coreutils, fetchgit, libuuid, czmq, openssl,
   gnuplot, fetchurl,
-
-  src ? ./.
+  src
 } :
 
 let