btrfs: remove last_ref from the extent freeing code

This is a remnant of the work I did for qgroups a long time ago to only
run for a block when we had dropped the last ref.  We haven't done that
for years, but the code remains.  Drop this remnant.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>