Delete/remove orphaned users and their schema from server/database
To find out the orphan users of a database say "ABC", you need to query below on that database:i.e

USE ABC

sp_change_users_login @Action='Report'

This will list out all the orphan database users for that database. In order to resolve the orphan users, use sp_change_users_login, with query below 

USE ABC

sp_change_users_login
    @Action='update_one',   
    @UserNamePattern='database_user_name_comes_here', 
    @LoginName='login_name_comes_here'



Also, suggested reads: How To Avoid Orphaned Database Users with SQL Server Authentication and Fixing Orphan users