repartition_largest_partition parameters are not optional

In the [original code][1] these parameters are not optional.

The [current test case][2] pass the parameters.

[Both][3] [scripts][4] pass the parameters.

The [C++ code][6] and [header][5] doesn't define defaults.

I rest my case.

[1]: https://github.com/ged-lab/khmer/commit/f05b5bbaedf314d3f6fa5b1df89c3902ca4119ee#diff-ace8ca86673d4c80b9a93a6f69ac7126R1622
[2]: https://github.com/ged-lab/khmer/blob/9e6474c88493cff59e8e47010a15a4944204a1e9/tests/test_lump.py#L94
[3]: https://github.com/ged-lab/khmer/blob/9e6474c88493cff59e8e47010a15a4944204a1e9/scripts/make-initial-stoptags.py#L91
[4]: https://github.com/ged-lab/khmer/blob/b40585817695550b4c7fda860eb1ed8f085b9841/scripts/find-knots.py#L93
[5]: https://github.com/ged-lab/khmer/blob/b40585817695550b4c7fda860eb1ed8f085b9841/lib/subset.hh#L139
[6]: https://github.com/ged-lab/khmer/blob/b40585817695550b4c7fda860eb1ed8f085b9841/lib/subset.cc#L1454