Remove unused code silencing intercept errors (#836)

Remove unused code silencing intercept errors (#836)

The removed code has not been actively used almost since it was added.

It is now widely accepted that NAT and TPROXY can only be done on the
machine running Squid. The corresponding address lookup errors are an
indication of either a system misconfiguration or an adverse external
event such as flushing of conntrack tables. Since these errors should be
fatal to the affected transactions and the admin usually has the power
to address them, Squid should report them at level 1.