btrfs: extent_io: Kill dead condition in extent_write_cache_pages()

Since __extent_writepage() will no longer return >0 value,
(ret == AOP_WRITEPAGE_ACTIVATE) will never be true.

Kill that dead branch.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>