MovePicker doesn't need to know if called from a pv node

MovePicker doesn't need to know if called from a pv node

This was needed by an old optimization in sorting of
non-captures that is now obsoleted by new std::sort()
approach.

Remove also the unused depth member data. Interestingly
this has always been unused since the Glaurung days.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>