Choose only one IOPlugin implementation

Right now, we have both DefaultTableIOPlugin and CommonsCSVTableIOPlugin. The default one wins, because it has a higher priority. The commons CSV one is never used.

One of these plugins should be deleted. But which one? Let's make a decision. The commons CSV implementation is substantially shorter and hence probably more maintainable. But which one is faster? Is the commons-csv dependency acceptable? Is either one more correct than the other, covering more cases? Etc.