Clean up unused code

It appears that [`IonTextBufferedStream`](https://github.com/amazon-ion/ion-java/blob/864f351bf7a8851ff264123d59fd52cee6d05c26/src/main/java/com/amazon/ion/impl/IonTextBufferedStream.java#L28)  is unused. We should go through and clean up this and any other unused code.
