repl: remove magic mode

The magic mode is long deprecated and works the same as the sloppy
mode. Since the sloppy mode is the default, removing the magic mode
should be safe.

PR-URL: https://github.com/nodejs/node/pull/19187


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>