lua: actually unobserve properties in mp.unobserve_property()

Not doing this looked like a memory leak.

This looks like an oversight in the commit that added it: a94020e25b,
possible brain damage?

Fixes: #6823

player/lua/defaults.lua

@@ -355,6 +355,7 @@ function mp.unobserve_property(cb)
     for prop_id, prop_cb in pairs(properties) do
         if cb == prop_cb then
             properties[prop_id] = nil
+            mp.raw_unobserve_property(prop_id)
         end
     end
 end