fix(win_split_ins): do not fail when oldwin is not valid

Ref #14240

src/nvim/quickfix.c

@@ -3646,15 +3646,6 @@ static int qf_open_new_cwindow(qf_info_T *qi, int height)
   if (win_split(height, flags) == FAIL) {
     return FAIL;  // not enough room for window
   }
-
-  // User autocommands may have invalidated the previous window after calling
-  // win_split, so add a check to ensure that the win is still here
-  if (IS_LL_STACK(qi) && !win_valid(win)) {
-    // close the window that was supposed to be for the loclist
-    win_close(curwin, false, false);
-    return FAIL;
-  }
-
   RESET_BINDING(curwin);
 
   if (IS_LL_STACK(qi)) {