binfmt/elf: Fix the minor style issue

and remove the unused macros and unnecessary cast

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

binfmt/builtin.c

@@ -96,7 +96,7 @@ static int builtin_loadbinary(FAR struct binary_s *binp,
    * the priority.  That is a bug and needs to be fixed.
    */
 
-  builtin         = builtin_for_index(index);
+  builtin = builtin_for_index(index);
   if (builtin == NULL)
     {
       berr("ERROR: %s is not a builtin application\n", filename);