Revert "steam: use custom openssl"

No longer necessary. See https://github.com/NixOS/nixpkgs/pull/23034#issuecomment-291005754

This reverts commit a120bad30cbd63019971e9d03edd82423131852c.

pkgs/games/steam/runtime-wrapped.nix

···
     SDL
     SDL2_image
     glew110
+    openssl
     libidn
 
     # Other things from runtime
···
     alsaLib
     openalSoft
     libva
-    openssl-steam
     vulkan-loader
   ] ++ lib.optional newStdcpp gcc.cc;