remove erroneous lead from test system

The lead does not have connected unit cells and serves no purpose.
We also remove the check for the number of 'collections' in the
resulting plots, as now both 'syst_2d' and 'syst_3d' produce
systems with 2 leads.

kwant/tests/test_plotter.py

@@ -59,13 +59,10 @@ def syst_2d(W=3, r1=3, r2=8):
     syst[lat.neighbors()] = -t
     sym_lead0 = kwant.TranslationalSymmetry(lat.vec((-1, 0)))
     lead0 = kwant.Builder(sym_lead0)
-    lead2 = kwant.Builder(sym_lead0)
 
     lead_shape = lambda pos: (-W / 2 < pos[1] < W / 2)
 
     lead0[lat.shape(lead_shape, (0, 0))] = 4 * t
-    lead2[lat.shape(lead_shape, (0, 0))] = 4 * t
-    syst.attach_lead(lead2)
     lead0[lat.neighbors()] = - t
     lead1 = lead0.reversed()
     syst.attach_lead(lead0)
@@ -111,8 +108,7 @@ def test_plot():
                 if (color != 'k' and
                     isinstance(color(next(iter(syst2d.sites()))), float)):
                     assert fig.axes[0].collections[0].get_array() is not None
-                assert len(fig.axes[0].collections) == (8 if syst is syst2d else
-                                                        6)
+                assert len(fig.axes[0].collections) == 6
         color_opts = ['k', (lambda site, site2: site.tag[0]),
                       lambda site, site2: (abs(site.tag[0] / 100),
                                            abs(site.tag[1] / 100), 0)]