Remove hardcode default values of UCI variables from evaluation

Remove hardcode default values of UCI variables from evaluation

This is the same change we have already done in search.cpp,
this time for evaluation.cpp

If a variable will be populated reading an UCI option
then do not hard code its default values.

This avoids misleadings when reading the sources.

No functional change.

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