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.