btrfs: remove unused flag EXTENT_BUFFER_CORRUPT

This flag is no longer being used.  It was added by commit a826d6dc
("Btrfs: check items for correctness as we search") but it's no longer
being used after commit f26c9238 ("btrfs: remove reada
infrastructure").

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>