refactor(aucmd_win): remove need to restore window layout

There are some places that mess with the window layout in preparation for moving
a window to a different split (win_split_ins called with new_wp != NULL).

This means the window layout can change slightly even if win_split_ins fails.

This is why it was still needed to restore the window layout in
aucmd_{prep,rest}buf even if we disallow win_split_ins from making aucmd_win
non-floating by moving it into a split.

We can just skip messing with the layout in such places if we're dealing with
the aucmd_win.

src/nvim/eval/funcs.c

@@ -4242,7 +4242,7 @@ static void win_move_into_split(win_T *wp, win_T *targetwin, int size, int flags
   int height = wp->w_height;
   win_T *oldwin = curwin;
 
-  if (wp == targetwin) {
+  if (wp == targetwin || wp == aucmd_win) {
     return;
   }