heimdal: remove checking of KDC PAC signature, delegate to wdc plugin

heimdal: remove checking of KDC PAC signature, delegate to wdc plugin

The checking of the KDC signature is more complex than it looks, it may be of a different
enc type to that which the ticket is encrypted with, and may even be prefixed
with the RODC number.

This is better handled in the plugin which can easily look up the DB for the
correct key to verify this with, and can also quickly determine if this is
an interdomain trust, which we cannot verify the PAC for.

Andrew Bartlett