Updating graphical defaults based on recommendations in Wilke's book

[IndrajeetPatil]

"For a plot where we are primarily interested in the change in y values, vertical grid lines are not needed." - C. Wilke

• The vertical grid lines in ggbetweenstats and ggwithinstats can be removed
• For the same reason with x values, the grid lines can be removed for ggcoefstats and ggdotplotstats
• The y-axis labels and grid are completely unnecessary for {ggbarstats} (for the same reason, {ggpiestats} doesn't have any axes)

[IndrajeetPatil]

PR #1069 addresses the first item from this issue: removing vertical grid lines in ggbetweenstats and ggwithinstats.

The other proposed changes (removing horizontal grid lines in ggcoefstats/ggdotplotstats, and removing y-axis labels/grid in ggbarstats) were explored but not retained, as they made it harder to visually assess data location in those plots.