[sled agent] Deprecate the old service format (#5922)

This portion of the Sled Agent was added in
#4466, migrating an old
"services" ledger into a new, zones-oriented format.

This code served as a bridge between old and new format, but maintaining
backwards compatibility here is a burden
for modifying the zone format further.

Specifically: in #5050 ,
which attempts to add the notion of "transient zone filesystem zpool" to
the request from Nexus, the translation of "old format -> new format" is
no longer possible without additional information about "where should
the transient zone filesystem be placed".

Given that this conversion has been deployed for multiple months -
across releases - I figured it would be easier to remove the old
support, rather than making another bridge for old code compatibility.

sled-agent/src/lib.rs

@@ -33,7 +33,6 @@ mod profile;
 pub mod rack_setup;
 pub mod server;
 pub mod services;
-pub mod services_migration;
 mod sled_agent;
 mod smf_helper;
 mod storage_monitor;