[Kotlin client] fix warning when else all covered

[karismann]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.
  @jimschubert (2017/09), @dr4ke616 (2018/08)

Description of the PR

fix #2462
remove else in when clause cause all responseType values are covered

[wing328]

Tested locally and the warnings no longer occurs.