Remove a spec for Array#{*,to_s}

This sharing spec is failing on Array#{*,to_s}.

* Array#* does not coerce nil as Array#join.
* Array#to_s on 1.8 does not accept arguments as Array#{join,*}.

But this spec is written as Array#{join,*,to_s} and "string separator".

https://github.com/rubinius/rubinius/commit/7fbc8dea310ed522b730113bd5abc5ae20cbda5f#commitcomment-3756639