Remove constant_hessian_value?

[NicolasHug]

I think we can drop the constant_hessian_value in the SplittingContext, and always assume the constant hessians value is 1. We just have to set the gradients value accordingly, to have consistent leaf values.

This is what we do for LS loss already. WDYT @ogrisel ?

[ogrisel]

As you wish. +0 to simplify the code a bit.