Remove highWatermark option

[ehmicky]

#121 added the on(stream, 'data')'s highWatermark option. However, I realized this does not make sense in this specific case.

That option is measured in number of data events, not in bytes. My PR assumed it was measured in bytes. Since we cannot know the average number of bytes per data event (i.e. per readable.push()), we cannot know the optimal value for this option.

Also, the point of get-stream is already to buffer all stream data. So this is not a big problem if on(stream, 'data') buffers it too: if some data is buffered by on(), it is not consumed yet. Once consumed, it will be buffered by get-stream instead. So using this option does not actually reduce memory consumption.

Finally, get-stream's processing of each chunk is synchronous and fairly fast, so having a backlog of data events is unlikely.