fix: ruff rule F841 (unused variable)

[thatlittleboy]

Related to Issue https://github.com/dsgibbons/shap/issues/21, and is a continuation of PR https://github.com/dsgibbons/shap/pull/25 when ruff was initially introduced to the project.

This PR aims to make the codebase F841-compliant ("Local variables assigned but unused").

For the variables with no clear purpose, I removed them. Some I chose to keep/comment away (somewhat arbitrary decision based on a feeling that it might be useful for future reference, feel free to tell me to delete them).

Dependencies: The hstack bug of Explanation makes an appearance here too. So PR #86 should be merged in first. ✅ DONE

[codecov[bot]]

Codecov Report

Merging #90 (64a283d) into master (2cabb07) will decrease coverage by 0.02%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   53.65%   53.63%   -0.02%     
==========================================
  Files          90       90              
  Lines       12981    12957      -24     
==========================================
- Hits         6965     6950      -15     
+ Misses       6016     6007       -9     

Impacted Files	Coverage Δ
shap/_explanation.py	55.68% <ø> (+0.02%)	:arrow_up:
shap/plots/_beeswarm.py	53.78% <ø> (+0.02%)	:arrow_up:
shap/plots/_image.py	73.04% <ø> (-0.47%)	:arrow_down:
shap/plots/_scatter.py	26.62% <0.00%> (ø)
shap/plots/_text.py	54.48% <ø> (-0.15%)	:arrow_down:
shap/plots/_violin.py	76.26% <ø> (+0.40%)	:arrow_up:
shap/utils/_general.py	64.82% <ø> (+0.32%)	:arrow_up:
shap/plots/_bar.py	56.33% <50.00%> (ø)
shap/explainers/_sampling.py	89.18% <100.00%> (+0.69%)	:arrow_up:
shap/plots/_waterfall.py	83.54% <100.00%> (+0.31%)	:arrow_up:

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