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 <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>
Reviewed-By: Brian White <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>