Remove constant variables from datasets generated via SBC_generator_brms

[martinmodrak]

As noted in #85, the diagonal of correlation matrix (and possibly some other constant parameters) gets included in the dataset. This then causes NA in ESS/Rhat and is generally not useful. Those parameters should be detected and removed from dataset$variables.

[hrlai]

This would be very useful for a Student-t model where we sometimes fix the degrees-of-freedom parameter :)