[ldaputils,ldapparser] correctly compute "deleted" users (closes #2542083) * in ldaputils.py, removal of an unneeded method (checking existence of an object in the ldap, but not using the configuration filters) * in the ldapparser, memoize the (filtered) search so as to serve both .process and .is_deleted * add a non regression test for the 'user-filter' source config entry + deletion

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