Remove Buffer.iterator

[jerch]

@Tyriar

Since the old linkifier is gone, Buffer.iterator (and subsequentially BufferStringIterator and Buffer.stringIndexToBufferIndex) is not used by any code anymore. I think it can be removed. Also it is not exposed to outside, furthermore I think that this Java-ish iterator was not the best idea. (In case we want a similar feature in the future, maybe it could be achieved more idiomatic by a generator pattern.)

[Tyriar]

Nice 👍. I think this was added before browsers supported generators well, or at least before I knew we were clear to use them.