Summary:
Addressed a few YB_TODOs in `main` in `pg_upgrade.c`:

=== `copy_xact_xlog_xid` ===

copy_xact_xlog_xid and start_postmaster were both wrapped in an `#ifdef YB_TODO` block.

`copy_xact_xlog_xid` copies Postgres commit and WAL data around. Any data that is actually required
by YB for data integrity should be in DocDB, which isn't impacted by the upgrade. Therefore it
should be safe to fully disable this function.

=== `start_postmaster` / `stop_postmaster` ===
Previous work on `pg_upgrade` had wrapped calls to `start_postmaster` or `stop_postmaster` with a
`!is_yugabyte_enabled()` check, as we don't need to stop / start the postmaster in the YB upgrade.

=== Cleaning up the old cluster ===
Because Yugabyte data resides on DocDB and is reused from pg11 to pg15, we don't need to remove it. The Upgrade RPCs will handle removing old masters during the upgrade finalize step, so there is nothing required to clean up the old cluster.
Jira: DB-14539

Test Plan:
Jenkins

```
./yb_build.sh release --cxx-test pg15_upgrade-test
```

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D40720