Remove OrderedDict

[taniokay]

Motivation

Resolve #4592

Description of the changes

• Replace existing OrderedDict with dict
• Fix related tests and docstrings.

[codecov-commenter]

Codecov Report

Merging #4838 (a65916e) into master (222194a) will increase coverage by 0.01%.
Report is 59 commits behind head on master.
The diff coverage is 93.33%.

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@@            Coverage Diff             @@
##           master    #4838      +/-   ##
==========================================
+ Coverage   89.52%   89.54%   +0.01%     
==========================================
  Files         195      197       +2     
  Lines       14667    14668       +1     
==========================================
+ Hits        13131    13134       +3     
+ Misses       1536     1534       -2     

Files Changed	Coverage Δ
optuna/samplers/_search_space/intersection.py	90.69% <0.00%> (-0.22%)	⬇️
optuna/importance/__init__.py	96.15% <100.00%> (-0.15%)	⬇️
optuna/importance/_base.py	94.11% <100.00%> (-0.09%)	⬇️
optuna/importance/_fanova/_evaluator.py	100.00% <100.00%> (ø)
optuna/importance/_mean_decrease_impurity.py	100.00% <100.00%> (ø)
optuna/integration/botorch.py	94.42% <100.00%> (-0.02%)	⬇️
optuna/integration/shap.py	100.00% <100.00%> (ø)
optuna/samplers/_grid.py	100.00% <100.00%> (ø)
optuna/search_space/intersection.py	100.00% <100.00%> (ø)
optuna/study/_dataframe.py	94.20% <100.00%> (ø)
... and 1 more

... and 7 files with indirect coverage changes

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[toshihikoyanase]

@Alnusjaponica @not522 Could you review this PR, please?

[Alnusjaponica]

LGTM. Thanks for your contribution.

[not522]

Thank you! LGTM!