Remove unnecessary typedef for na_md_loop_t

Author: kojix2

ext/numo/narray/ndloop.c

@@ -38,8 +38,6 @@ typedef struct NA_LOOP_XARGS {
     bool free_user_iter;   // alloc LARG(lp,j).iter=lp->xargs[j].iter
 } na_loop_xargs_t;
 
-typedef struct NA_MD_LOOP na_md_loop_t;
-
 typedef struct NA_MD_LOOP {
     int  narg;
     int  nin;