btrfs: Remove EXTENT_WRITEBACK

This flag was introduced in a52d9a80 ("Btrfs: Extent based page
cache code.") and subsequently it's usage effectively was removed by
1edbb734 ("Btrfs: reduce CPU usage in the extent_state tree") and
f2a97a9d ("btrfs: remove all unused functions"). Just remove it,
no functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>