Remove vendored spec/support/rubocop_helper.rb

What does this MR do?

Removes vendored spec/support/rubocop_helper.rb, which was originally introduced in !2965 (merged), since Rubocop::Cop::AvoidUsingEnv cop is general one, not only for rspec, so we should expect example_spec.rb in its tests.

Blocks !5048 (merged)

Related issues

Closes #6007 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened