Remove y-axis for plots where y-axis has no purpose

This would technically break visual backwards compatibility although wouldn't affect anyone's code. @avehtari pointed out that for some plots we have a y-axis that serves no purpose and the plots look better without it. For example, the new `mcmc_dots()` function doesn't include the y-axis for this reason, but functions like `mcmc_dens()` and others include the y-axis for no reason. 