Remove implementation of __getattr__ for UniSchema

[v01dXYZ]

Hello, I found a very minor issue (probably a typo): https://github.com/uber/petastorm/blob/d6f4e82eb2c3a6c2b4c16c060c7350331b60a51a/petastorm/unischema.py#L355-L356 This is not really required (basically since we don't reimplement __getattribute__, that will always be object.__getattribute__(obj, item) that will be invoked when we getattr).

[selitvin]

Thanks for the report. Will clean it up (or feel free to submit a fix in a PR :) ).