remove unused argument in superconductor tutorial

doc/source/code/figure/superconductor.py.diff

@@ -28,7 +28,7 @@
  
  #HIDDEN_BEGIN_nbvn
  def make_system(a=1, W=10, L=10, barrier=1.5, barrierpos=(3, 4),
-                 mu=0.4, Delta=0.1, Deltapos=4, t=1.0, phs=True):
+                 mu=0.4, Delta=0.1, Deltapos=4, t=1.0):
      # Start with an empty tight-binding system. On each site, there
      # are now electron and hole orbitals, so we must specify the
      # number of orbitals per site. The orbital structure is the same