Clean up unitcell setting when `None` in MMTF reader

[IAlibay]

Follow up from #3132

See comment: https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r629694435

We should be able to clean this up a little bit.

[IAlibay]

This was me misreading the code. unit_cell can be and is set by mmtf:

https://github.com/rcsb/mmtf-python/blob/c913a6fc0194e50a825e86d7584ae2bbad3c1d7d/mmtf/api/mmtf_reader.py#L125-L128