repl: remove magic mode semantics

The workaround used in repl to support `let` and `const` in non-strict
mode (known as "magic" mode) has been unnecessary since V8 v4.9 /
Node.js v6.0.0. This commit functionally removes this workaround.

PR-URL: https://github.com/nodejs/node/pull/11599
Refs: https://v8project.blogspot.com/2016/01/v8-release-49.html
Refs: https://github.com/nodejs/node/blob/master/doc/changelogs/CHANGELOG_V6.md#6.0.0


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>