Remove libkdb_ldap null argument checks

Remove libkdb_ldap null argument checks

libkdb_ldap contains some spotty null argument pointer checks, which
is not the project's practice outside of the GSSAPI.  Remove them.
Also remove krb5_ldap_delete_krbcontainer(), which became unused after
commit 0269810b1aec6c554fb746433f045d59fd34ab3a.