Remove ipx copy-db (#85)

It was the one-off copy from SQLite to Postgres (#18), run once on 2026-09-18. Production has run
on Postgres since; rolling back needs only the old state.db, which is kept, not this command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/configuration.md

@@ -25,8 +25,7 @@ config.toml's default location is `$XDG_CONFIG_HOME/ipx/config.toml`
 
 `~` is expanded in paths. The database is SQLite in WAL mode unless `IPX_DATABASE_URL` names a
 Postgres database instead. Back SQLite up by copying `state.db` while the daemon is stopped, or
-with `sqlite3 state.db .backup`; back Postgres up with `pg_dump`. `ipx copy-db <state.db>` copies a
-SQLite database into the empty Postgres one `IPX_DATABASE_URL` names.
+with `sqlite3 state.db .backup`; back Postgres up with `pg_dump`.
 
 ## `[general]`