settings: clear filename after deleting file from disk

It doesn't really matter, because in the next step we
are about to remove the connection.

However, once the connection is deleted from file, it's
clear that it has no more file-name.

(cherry picked from commit 1fd5c52eaf)

src/settings/nm-settings-connection.c

@@ -755,6 +755,8 @@ nm_settings_connection_delete (NMSettingsConnection *self,
 	                    error))
 		return FALSE;
 
+	nm_settings_connection_set_filename (self, NULL);
+
 	set_visible (self, FALSE);
 
 	/* Tell agents to remove secrets for this connection */