Remove unused dimensions lines in PQR reader

[IAlibay]

Fixes #3327

Changes made in this Pull Request:

• Removes unused dimensions/unitcell lines
• Adds note clarifying that dimensions are never set
• Adds simple test

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated? - unecessary (not really a user facing change)
• Issue raised/referenced?

[pep8speaks]

Hello @IAlibay! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-31 19:28:51 UTC

[codecov]

Codecov Report

Merging #3378 (2f5fb15) into develop (6531904) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #3378   +/-   ##
========================================
  Coverage    93.62%   93.62%           
========================================
  Files          176      176           
  Lines        22839    22835    -4     
  Branches      3225     3224    -1     
========================================
- Hits         21382    21379    -3     
+ Misses        1406     1405    -1     
  Partials        51       51           

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/PQR.py	95.23% <ø> (+0.91%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6531904...2f5fb15. Read the comment docs.