eradicate last remains of default_site_group

doc/source/tutorial/tutorial2.rst

@@ -318,11 +318,6 @@ period of one flux quantum.
 
 .. specialnote:: Technical details
 
-  - Note that in this example, we did not need to set
-    ``sys.default_site_group = lat``. All lattice points were
-    added using functionality from ``lat`` and thus were
-    proper sites already.
-
   - Leads have to have proper periodicity. Furthermore, the kwant
     format requires the hopping from the leads to the scattering
     region to be identical to the hoppings between unit cells in

kwant/solvers/tests/_test_sparse.py

@@ -362,7 +362,6 @@ def test_ldos(ldos):
     sys = kwant.Builder()
     gr = kwant.lattice.chain()
     lead = kwant.Builder(kwant.TranslationalSymmetry(gr.vec((1,))))
-    sys.default_site_group = lead.default_site_group = gr
     sys[gr(0)] = sys[gr(1)] = lead[gr(0)] = 0
     sys[gr(0), gr(1)] = lead[gr(0), gr(1)] = 1
     sys.attach_lead(lead)