REF: simplify calculation of Simpson diversity

[martinfleis]

I realised that the simpson_diversity helper function can be much simpler than it was. Also, categories keyword made no difference to the calculation so I dropped it from the helper. It is still part of the signature of Simpson though, to avoid any breakage. I assume that it will get cleaned with the API revamp (#308).

[codecov]

Codecov Report

Merging #320 (1b6a816) into main (8d341e5) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #320      +/-   ##
==========================================
+ Coverage   95.97%   95.99%   +0.02%     
==========================================
  Files          13       13              
  Lines        2832     2822      -10     
==========================================
- Hits         2718     2709       -9     
+ Misses        114      113       -1     

Impacted Files	Coverage Δ
momepy/diversity.py	98.00% <100.00%> (+0.30%)	⬆️

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