Allow subgroup deletion total-pages is unknown

What does this MR do and why?

The x-total-pages header isn't returned if there are more than 10000 records, which is currently the case in https://gitlab.com/gitlab-qa-sandbox-group, causing a test to fail when it tries to filter a list of all the subgroups: #341930 (closed).

The x-next-page header is always returned, so we'll use that instead and stop fetching pages when there's no longer a next one.

How to set up and validate locally

export GITLAB_ADDRESS=https://gitlab.com
export GITLAB_QA_ACCESS_TOKEN=<PAT for user with access to gitlab-qa-sandbox-group>
bundle exec rake delete_subgroups

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