Previously, setting/deleting the "--type" virtual attribute removed
all (objectclass=...) target filters.
Change so that only the filter associated with --type is removed.
The same change applies to --memberof: only filters associated
with the option are removed when --memberof is (un-)set.
Follow-up to https://fedorahosted.org/freeipa/ticket/4216
Reviewed-By: Martin Kosek <mkosek@redhat.com>