unused variable in beta.hpp

[Ben Goodrich]

git grep -n log_beta_AB src/

shows that

src/stan/prob/distributions/univariate/continuous/beta.hpp:123: const double log_beta_AB = log(boost::math::beta(alpha_dbl, beta_dbl));

is not used elsewhere in that function. Can we get rid of log_beta_AB or was it supposed to be used in the gradient calculation somewhere?

Ben

[Bob Carpenter]

The doc implies it's for gradients, but
it's clearly not used anywhere, so by all
means nuke it.

I think what happened is that we broke the
beta function down into its component gamma
functions to separate what's needed for normalization
where.

- Bob