remove .flatten on standard filters

Standard filter should not flatten arrays.

We should not be forcing a flatten on standard filter. this was added 5 years ago 36860ea and I think was a mistake. Maybe was meant to be same as `Array(input)`.
Anyhow, we should remove flatten because liquid should not change the input given.

review @fw42 @jamesmacaulay 
cc @boourns 

please cc anyone else that should be in this.

To be honest, I don't think we need this new flatten filter at all... I can't think of a use-case for it.