This was leftover from trying to hold the invariant of one empty draft,
which was dropped in #53852.

Removes all automatic draft deletion:

- `retain_running_thread` discarded empty drafts when navigating away
- `remove_empty_draft` cleaned up all retained drafts when loading from
history
- `open_external_thread_with_server` removed empty drafts when opening
external threads

Release Notes:

- Fixed agent drafts being silently discarded when navigating between
threads