Drop upstream's issuer-routing test and an import it left unused

tests/src/directory/issuer.rs, new in v0.16.23, tests routing a bearer token
to a directory by its issuer. That routing is Enterprise-only upstream (the
body of get_directory_for_issuer), and the fork doesn't build it: a token
naming no address gets the server default (DIR-2). The test also calls a
helper from upstream's Enterprise-only OIDC test, so it can't compile here.

mta.rs imported types::id::Id for code inside an Enterprise snippet; the
stripped tree leaves it unused, upstream's as well as ours.

crates/common/src/network/mta.rs

@@ -38,7 +38,6 @@ use store::{
     write::{AlignedBytes, Archive, QueueClass, ValueClass},
 };
 use trc::{AddContext, SpamEvent};
-use types::id::Id;
 use utils::DomainPart;
 
 impl Server {