Clean up use of `unitcell` in PQR

[IAlibay]

Follow up from #3132 (https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r632672743)

The way we use unitcell in the PQRReader isn't super consistent given that we really only set it to None. Given the way the format works (https://github.com/MDAnalysis/mdanalysis/pull/3132/files#r629707068), I think we can just safely remove any code paths that assume non-empty unitcell values.

[IAlibay]

For context, this is the conversation I am referring to here:

from @mnmelo (replying to a question for me re: if unitcell is ever set):

"Just tried pdb2pqr, which seems to be sort of a reference on PQR, on a well-formed PDB with meaningful CRYST1 field. The output has no CRYST1 or box info whatsoever, so we probably shouldn't expect any when reading.

I agree this could be simplified to a single line."