feat: removing duplicate implementation of Dutch National Flag Sort Algorithm

[sailok]

Dutch National Flag Sort Algorithm has been implemented at two places:

• https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/sorts/DNFSort.java
• https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/sorts/DutchNationalFlagSort.java

Removed the prior implementation in favour of the second one.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• 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 ✅

Project coverage is 52.80%. Comparing base (d436910) to head (277860f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5539      +/-   ##
============================================
+ Coverage     52.71%   52.80%   +0.08%     
+ Complexity     3305     3304       -1     
============================================
  Files           529      528       -1     
  Lines         15384    15354      -30     
  Branches       2926     2924       -2     
============================================
- Hits           8110     8107       -3     
+ Misses         6941     6912      -29     
- Partials        333      335       +2     

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