Remove unused `isArray` polyfill

[kenany]

Not being used anywhere. Or, should it be used here?

[shinnn]

:+1: for just removing it. Users can add another isArray polyfill as needed.

[stevemao]

:+1:

[ljharb]

better to use the isarray module.

[eagleeye]

@maxogden @mafintosh how about merging this?

[ljharb]

https://github.com/maxogden/concat-stream/pull/32#issuecomment-210683044

[kenany]

Could also do that if desired (note, that specific module is deprecated, but there are others).

[ljharb]

That deprecation only applies to v2, and is simply a paternalistic way of trying to push people not to support ES3. You can and should use v1 of that package for the foreseeable future.