cubicweb #2936496 drop ldapuser source [validation pending]

ldapfeed is a full replacement to ldap user. This drop have been discussed and approved several times.

priorityimportant
typeenhancement
appeared in<not specified>
done in3.18.0
load0.250
load left0.000
closed by#9504:65b8236e1bb4
patch[sources] drop support for ldapuser source (closes #2936496) [applied]