Drop unused deprecation warning silence

What does this MR do?

This removes a Ruby 3 related warning silence in DeprecationEnvToolkit we had put in place for gitlab-experiment. It had been detected as part of the Ruby 3 upgrade work, was originally fixed in https://gitlab.com/gitlab-org/gitlab-experiment/-/merge_requests/124 and merged back into GitLab as part of the 0.6.4 release: https://gitlab.com/gitlab-org/gitlab-experiment/-/merge_requests/127

So we do not need it anymore.

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