Logout from Community Portal using apex

Please use below code to redirect to community login page :

window.location.replace("<community-domain>/secur/logout.jsp?retUrl=<redirect-URL>");

Eg: 

1)  window.location.replace("/testPortal/secur/logout.jsp?retUrl=%2Flogin");

2)  window.location.replace("https://mycommunity-domain.com/secur/logout.jsp?retUrl=https%3A%2F%2Fmycommunity-domain.com%2Flogin");