cleanup combination and combination test

[lorduke22]

Related to https://github.com/TheAlgorithms/Java/issues/5164

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.29%. Comparing base (b5a097c) to head (37687e0).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5902      +/-   ##
============================================
- Coverage     72.30%   72.29%   -0.01%     
- Complexity     4956     4957       +1     
============================================
  Files           650      650              
  Lines         17546    17548       +2     
  Branches       3383     3384       +1     
============================================
  Hits          12687    12687              
- Misses         4382     4383       +1     
- Partials        477      478       +1     

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