[KOTLIN-SPRING] - remove springfoxDocumentationConfig.mustache

[Picazsoo]

remove the file as it is not used anywhere and we are not supporting springfox anymore.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
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• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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Summary by cubic

Removed springfoxDocumentationConfig.mustache from the kotlin-spring generator because springfox is no longer supported. This removes a dead template and avoids generating unused Springfox config.

^{Written for commit 1336e1c. Summary will update on new commits.}

[cubic-dev-ai]

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