Removed hidden argument to set_targets_json_location

The previous code used a variable that was internal to the Target class.
This commit removes the argument to `set_targets_json_location`
completely, which forces Target to use the default locatio internally.

Author: Bogdan Marinescu

tools/test/config_test/config_test.py

@@ -44,7 +44,7 @@ def test_tree(full_name, name):
     if os.path.isfile(os.path.join(full_name, "targets.json")):
         set_targets_json_location(os.path.join(full_name, "targets.json"))
     else: # uset the regular set of targets
-        set_targets_json_location(Target._Target__targets_json_location_default)
+        set_targets_json_location()
     for target, expected in test_data.expected_results.items():
         sys.stdout.write("%s:'%s'(%s) " % (name, expected["desc"], target))
         sys.stdout.flush()