refactor: remove some unused includes (#19747)

- Remove autocmd.h from fileio.h
- Remove normal.h from main.h
- Move bufinfo_T from undo_defs.h to undo.c

src/nvim/ops.c

@@ -24,7 +24,6 @@
 #include "nvim/ex_cmds2.h"
 #include "nvim/ex_getln.h"
 #include "nvim/extmark.h"
-#include "nvim/fileio.h"
 #include "nvim/fold.h"
 #include "nvim/getchar.h"
 #include "nvim/globals.h"