Fix undefined behaviour in update.py

[ErikDanielsson]

There were some undefined behaviour in update.py in a recently merged pull request as @fabianegli pointed out in #1738 (comment). This fixes the issues mentioned there. I also took the opportunity to remove the DiffEnum class update.py which is not used any more.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1742 (1eebfa0) into dev (b5f7365) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev    #1742      +/-   ##
==========================================
- Coverage   68.47%   68.45%   -0.03%     
==========================================
  Files          59       59              
  Lines        7141     7136       -5     
==========================================
- Hits         4890     4885       -5     
  Misses       2251     2251              

Impacted Files	Coverage Δ
nf_core/modules/update.py	75.64% <66.66%> (-0.45%)	⬇️
nf_core/utils.py	80.97% <0.00%> (ø)

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[fabianegli]

Thank you for taking DiffEnum out, that was bugging me since I started working on tools 🙄