Remove policy scripts

Rhai made a decision about a route cost one to three microseconds,
some two hundred times what the same costs in Rust. That was bearable
where a function is asked once for an UPDATE, and is not where every
route is weighed against every peer, as it is when routes are passed
on. What is wanted of a policy (mark routes by where they came from,
mark what is announced, let communities say who is told what) needs no
language; it is to be configuration, done natively.

Kept from that work: route selection goes by a LOCAL_PREF that policy
set on a route from an external peer, and the lab's configuration can
be added to by a phase.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/integration-tests/tests/negotiation.rs

@@ -193,8 +193,6 @@ async fn test_no_family_in_common_refuses_the_session() -> Result<()> {
             import: ImportPolicy::permissive(),
             role: None,
             require_role: false,
-            import_hook: None,
-            export_hook: None,
         }];
         // Keep the dialer out of the way.
         c.connect_retry = Duration::from_secs(3600);