There are many system tests where we set `dnssec-validation yes;` only
to also set `trust-anchors { };` which effectively disables the
validation.
This commit replaces this convoluted setup with just
`dnssec-validation no;`.
Explicitly use an empty 'trust-anchors' statement in the system
tests where it was used implicitly before.
In resolver/ns5/named.conf.in use the trust anchor in 'trusted.conf',
which was supposed to be used there.
