otel: drop legacy profile usage telemetry (#24061)

## Summary
- drop the dead legacy profile usage metric and active-profile
conversation-start fields
- update role comments so they describe provider and service-tier
preservation without legacy config-profile wording
- pair the code cleanup with the file-backed profile docs update in
openai/developers-website#1476

## Testing
- `just fmt`
- `cargo test -p codex-otel`
- `cargo test -p codex-core` *(fails: existing stack overflow in
`mcp_tool_call::tests::guardian_mode_mcp_denial_returns_rationale_message`)*
- `cargo test -p codex-core --lib
mcp_tool_call::tests::guardian_mode_mcp_denial_returns_rationale_message`
*(fails with the same stack overflow)*

codex-rs/core/src/session/session.rs

@@ -816,7 +816,6 @@ impl Session {
                     .permissions
                     .legacy_sandbox_policy(session_configuration.cwd.as_path()),
                 mcp_servers.keys().map(String::as_str).collect(),
-                /*active_profile*/ None,
             );
 
             let use_zsh_fork_shell = config.features.enabled(Feature::ShellZshFork);