Remove json cfg after test

Otherwise CI thinks this is an openPMD file

Author: franzpoeschel

test/python/unittest/API/APITest.py

@@ -2239,6 +2239,8 @@ def testSeriesConstructors(self):
                 s = io.Series(f, io.Access.create, c)
                 s.close()
 
+        os.remove(cfg_as_file)
+
     def testScalarHdf5Fields(self):
         if "hdf5" not in io.variants:
             return