Removed max depth condition on the cache lookup

The intention probably was avoiding a lookup that will miss anyway as at
max depth (maximal value of `d' - minimal actual depth) we thought we
wouldn't have visited the position before. However

1. Missing is fine, we are not saving much
2. The code got a bit simplified again
3. It is possible, that we visit the same node again at root, the
   assumption was just simply incorrect.