[Golang][client] delete sample output dir before rebuild

Created by: grokify

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 and ./bin/security/{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, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

@antihax @bvwells

Description of the PR

This deletes the sample Go client output dir samples/client/petstore/go/go-petstore before rebuilding the sample with bin/go-petstore.sh.

Benefits:

1. Improves confidence in tests because they are run against a clean build without extra files
2. Keeps sample output dir clean and up to date since the output always reflects the current generator
3. Simplifies the process for contributors since there's no need to decide whether no longer generated files are important or not

Of note, this PR currently results in the deletion of the Swagger 2.0 spec at:

samples/client/petstore/go/go-petstore/api/swagger.yaml

The 3.0.1 OpenAPI spec remains at:

samples/client/petstore/go/go-petstore/api/openapi.yaml

If we want to keep the Swagger 2.0 spec, it seems like may be worth looking into the postProcessSupportingFileData in AbstractGoCodegen.java and related methods in DefaultGenerator.java.