Removes LDAP dependency per DDR-2563

NOTE: This implementation changes the behavior of Ddr::Auth::DetachedAuthContext with respect to affiliation. There is not a convenient way other than LDAP to get the list of all affiliation values for a Duke user. The Streamer API (streamer.oit.duke.edu), for example, only provides primary affiliation. However, since DetachedAuthContext would generally be used to run privileged operations like batch processes, the loss of affiliation is not likely to present practical issues.