`sprand(d::GaussianJacobi, n::Integer, a::Real, b::Real)` is redundant

The arguments `a` and `b` aren't needed because `GaussianJacobi` types have the fields `d.a` and `d.b`.

Locate the implementation and call sites for `sprand(d::GaussianJacobi, n::Integer, a::Real, b::Real)`, then inspect how `GaussianJacobi` exposes `d.a` and `d.b`. Remove the redundant arguments consistently and update any affected tests or callers; done means the API uses the distribution fields without changing behavior.

索引モデルが issue の本文から書いたものです。