bcachefs: Remove duplicate included headers

bcachefs: Remove duplicate included headers

The header files dirent_format.h and disk_groups_format.h are included
twice. Remove the redundant includes and the following warnings reported
by make includecheck:

  disk_groups_format.h is included more than once
  dirent_format.h is included more than once

Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>