Scaling layer none - Redundant calculation and warning of STD

[leondavi]

The scaling layer none case should move outside the if statement that performs std calculation.

If user selects the "layer none" it means that he already takes care to the scaling process of data, therefore, there is no need to calculate the std nor display a warning message in case std is smaller than numeric limits.

opennn/opennn/scaling_layer_2d.cpp

Line 874 in 0e5f526

if(scaler == Scaler::NoScaling)