repl: remove workaround for function redefinition

The issue is fixed upstream in V8. Thus we do not need this workaround
in REPL.

Fixes: https://github.com/nodejs/node/issues/548
PR-URL: https://github.com/nodejs/node/pull/9618


Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>