Use DegreeOverPrimeField to simplify some code

[fingolfin]

Also remove some dead code, and switch one place to use NrRows/NrCols.

[codecov[bot]]

Codecov Report

Merging #37 (486599d) into master (b1182fb) will increase coverage by 0.01%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   80.36%   80.37%   +0.01%     
==========================================
  Files           5        5              
  Lines        3606     3603       -3     
==========================================
- Hits         2898     2896       -2     
+ Misses        708      707       -1     

Files Changed	Coverage Δ
lib/forms.gi	74.06% <75.00%> (+0.01%)	:arrow_up:
lib/recognition.gi	79.09% <100.00%> (ø)