Remove non_test group specification on bundle install

Summary

!322 (merged) introduced non_test group to Gemfile by @Ahmadposten (around %11.1), but very soon after it, it was removed in !404 (merged) (%11.2).

Why

This change would eliminate unnecessary deprecation warning from the CI job log:

$ bundle install -j $(nproc) --without non_test
[DEPRECATED] The `--without` flag is deprecated because it relies on being remembered across bundler invocations, which bundler will no longer do in future versions. Instead please use `bundle config set without 'non_test'`, and stop using this flag

https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/1126027482