btrfs: remove unused flag EXTENT_BUFFER_READAHEAD

This flag is no longer being used.  It was added by commit ab0fff03
("btrfs: add READAHEAD extent buffer flag") and used in commits:

79fb65a1 ("Btrfs: don't call readahead hook until we have read the entire eb")
78e62c02 ("btrfs: Remove extent_io_ops::readpage_io_failed_hook")
371cdc07 ("btrfs: introduce subpage metadata validation check")

Finally all the code using it was removed by 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>