ZFS: remove one OSv ifdef

For every OSv specific ifdef we remove in ZFS, God ressurects a kitten.

After some recent additions and fixes, that piece of code can now be
compiled out. It is the reclaimer code, so it is very welcome.

But beware: that does not means we are reclaiming yet. That only means that we
wired up the ZFS ARC reclaiming process to the BSD notification system.

We now need to somehow wire that notification system with the OSv shrinking
infrastructure. That is the easy part. And after that, of course, balance the
calls between ARC and balloon. That is the hard part.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>