benchmark: remove broken string-creation.js

Strings where never mutable, it is not clear what this benchmarks
attempts to do. This did work at some point, but only because the
benchmark wasn't using strict mode.

PR-URL: #7094
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>