Remove top/right spines from "ticks" style

[mwaskom]

Would have done this originally if the rcParams existed. Now they do! So it would be a good idea.

Note that the aesthetics tutorial should change because it currently demonstrates despine using ticks, which won't make any sense after that.

[stefmolin]

A couple of questions on this one:

1. Do you mean editing lines like these in rcmod.py? Then, if someone calls sns.set_theme() before plotting, even with despine=False there will be no top or right spines displayed.
2. For the aesthetics guide, is it just the first example in this section?