remove useless debug message when executing a command

As we return the error message if the execution fail, use it from where
the method is called.

daemon/core/core.go

@@ -25,7 +25,6 @@ func Exec(executable string, args []string) (string, error) {
 
 	raw, err := exec.Command(path, args...).CombinedOutput()
 	if err != nil {
-		fmt.Printf("ERROR: path=%s args=%s err=%s out='%s'\n", path, args, err, raw)
 		return "", err
 	} else {
 		return Trim(string(raw)), nil