Explicitly use delta psqt values when possible

Explicitly use delta psqt values when possible

Instead of add and subtract pqst values corrisponding to
the move starting and destination squares, do it in one
go with the helper function pst_delta<>()

This simplifies the code and also better documents that what
we need is a delta value, not an absolute one.

No functional change.

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