Remove unused retrofit v1 mustache templates

[Philzen]

These are not used by any generator implementation, hence there's no point in bundling them into every release we're shipping.

In case someday it is decided that a retrofit v1 generator will be needed again, this commit can simply be reverted.

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 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

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  Commit all changed files.
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