[chore][elixir] delete old sample file

[efcasado]

Description

Delete an old sample file that prevents us from compiling the samples generated by the Elixir generator.

Running

mix deps.get
mix deps.compile
mix compile

results in

== Compilation error in file lib/openapi_petstore/model/deprecated_object.ex ==
** (ArgumentError) could not load module Jason.Encoder due to reason :nofile, cannot derive Jason.Encoder for OpenapiPetstore.Model.DeprecatedObject
    (elixir 1.18.3) lib/protocol.ex:325: Protocol.assert_protocol!/2
    (elixir 1.18.3) lib/protocol.ex:989: Protocol.derive/4
    (stdlib 6.2.2) lists.erl:2310: :lists.foreach_1/2
    (elixir 1.18.3) lib/protocol.ex:982: Protocol.__derive__/3
    lib/openapi_petstore/model/deprecated_object.ex:10: (module)

Mind the reference to Jason.Encode, which was removed in #21039.

It seems this sample file was last modified over a year ago using OpenAPI generator 7.0.0. That is, this sample is not being updated when we run

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

and the CI/CD pipelines are not detecting the drift. Thus, I suspect it's a file that we no longer need to track and can be safely deleted.

@mrmstn, @wing328 what do you think?

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 Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• 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)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.