Remove client listener callback

[pnuu]

In a continued effort to make Trollmoves easier to maintain, this PR removes the callback mechanism from the Client. In the same go, the unused trollmoves.client.StatCollector is removed.

• Tests updated

[codecov]

Codecov Report

Merging #103 (c8eea22) into main (3dd2584) will increase coverage by 0.22%.
The diff coverage is 97.59%.

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
+ Coverage   76.88%   77.10%   +0.22%     
==========================================
  Files          18       18              
  Lines        3526     3508      -18     
==========================================
- Hits         2711     2705       -6     
+ Misses        815      803      -12     

Flag	Coverage Δ
unittests	77.10% <97.59%> (+0.22%)	⬆️

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Impacted Files	Coverage Δ
trollmoves/client.py	72.26% <85.71%> (+0.85%)	⬆️
trollmoves/tests/test_client.py	99.53% <100.00%> (+<0.01%)	⬆️

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[mraspaud]

LGTM!