HPCC-12598 Configgen ldapserver enumeration map incorrectly updated

When "-listldapservers" specified in configgen, there is some code in the
ldapServers string map that incorrectly removes an entry after it has been
processed. Currently it sets that value to NULL, when in reality it should
be calling the remove() method. This fix changes that to use remove

Signed-off-by: William Whitehead <william.whitehead@lexisnexis.com>