On Windows we don't need anything to open a file

On Windows we don't need any command to open a file. 
And "start" has problems with spaces in the filename, even if we put quoter around them:
https://stackoverflow.com/questions/13691827/opening-file-with-spaces-in-windows-via-command-prompt

Author: supertobi

lib/dtutils/system.lua

@@ -122,12 +122,12 @@ dtutils_system.libdoc.functions["launch_default_app"] = {
 function dtutils_system.launch_default_app(path) 
   local open_cmd = "xdg-open "
   if (dt.configuration.running_os == "windows") then
-    open_cmd = "start "
+    open_cmd = "" -- On Windows we don't need any command. (start e.g. has problems with spaces in the filename, even if we put quoter around them.) https://stackoverflow.com/questions/13691827/opening-file-with-spaces-in-windows-via-command-prompt
   elseif  (dt.configuration.running_os == "macos") then
     open_cmd = "open "
   end   
   return dtutils_system.external_command(open_cmd .. path)
 end
 
 
-return dtutils_system
+return dtutils_system