Util: Removed duplicated isArray check

The extra check for typeof and toString is duplicating the work that `Array.isArray` already does (http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.3.2 and https://github.com/v8/v8/blob/8ae7cef19427362d2fb1c1fa4951050df6fe2e61/src/array.js#L1583).  As a result, any check to `util.isArray` that isn't an array will be checked twice.

Let me know if this should be deprecated and I will update the PR.