remove test file name

Author: davidhassell

cfdm/test/test_groups.py

@@ -501,7 +501,6 @@ def test_groups_unlimited_dimension(self):
         # ------------------------------------------------------------
         f.nc_set_variable_groups(["forecast", "model"])
 
-        grouped_file5 = "grouped_file5.nc"
         grouped_file = grouped_file5
 
         cfdm.write(f, grouped_file5)