Remove unused 'isolated' parameter

Now that `ConfigFileFinder.ignore_config_files` attribute is used
everywhere and is constructed from the `--isolated` CLI option, the now
unused `isolated` parameters can be safely removed.

tests/unit/test_get_local_plugins.py

@@ -7,8 +7,9 @@ from flake8.options import config
 def test_get_local_plugins_respects_isolated():
     """Verify behaviour of get_local_plugins with isolated=True."""
     config_finder = mock.MagicMock()
+    config_finder.ignore_config_files = True
 
-    local_plugins = config.get_local_plugins(config_finder, isolated=True)
+    local_plugins = config.get_local_plugins(config_finder)
 
     assert local_plugins.extension == []
     assert local_plugins.report == []

tests/unit/test_legacy_api.py

@@ -31,10 +31,10 @@ def test_get_style_guide():
 
     application.assert_called_once_with()
     mockedapp.parse_preliminary_options.assert_called_once_with([])
-    mockedapp.find_plugins.assert_called_once_with(config_finder, None, False)
+    mockedapp.find_plugins.assert_called_once_with(config_finder, None)
     mockedapp.register_plugin_options.assert_called_once_with()
     mockedapp.parse_configuration_and_cli.assert_called_once_with(
-        config_finder, None, False, [])
+        config_finder, None, [])
     mockedapp.make_formatter.assert_called_once_with()
     mockedapp.make_guide.assert_called_once_with()
     mockedapp.make_file_checker_manager.assert_called_once_with()

tests/unit/test_merged_config_parser.py

@@ -145,9 +145,10 @@ def test_merge_user_and_local_config(optmanager, config_finder):
 def test_parse_isolates_config(optmanager):
     """Verify behaviour of the parse method with isolated=True."""
     config_finder = mock.MagicMock()
+    config_finder.ignore_config_files = True
     parser = config.MergedConfigParser(optmanager, config_finder)
 
-    assert parser.parse(isolated=True) == {}
+    assert parser.parse() == {}
     assert config_finder.local_configs.called is False
     assert config_finder.user_config.called is False