Fix purgekeys -all for LDAP backend

We cannot pass NULL as the ber_values for
(LDAP_MOD_REPLACE | LDAP_MOD_BVALUES); conceptually, if we are
trying to LDAP_MOD_REPLACE with no key material, we really want
to just LDAP_MOD_DELETE the whole attribute (by passing NULL).

This feature appears to have never worked with the LDAP backend
since its introduction in release 1.12.

ticket: 8041
tags: pullup
target_version: 1.13.1
Regression tests for keyless principals

Confirm that kadmind does not crash when creating/modifying a principal
to have no keys, and confirm that no keys are present after a
purgekeys -all.
Remove some dead code

The secretkey variable is initialized to NULL and compared against
NULL, but never actually set to anything after initialization.

Remove the variable and all code that would have executed if it
was non-NULL.