Remove `show=...` argument from Kwant plotter

Matplotlib's own plotting functions have a separate logic for when to show figures or not. As far as I understand, we don't gain anything by adding a show argument to the plots. At the same time, because the figures we create are within pyplot's figure registry, we now may do things like

kwant.plot(syst, show=False)
pyplot.title('My awesome system')

If we remove show, the extra argument will not be needed. The downside is that the users running a non-interactive session will need to call pyplot.show() manually. This is consistent with how matplotlib works.