Remove Gitlab::Regex::BulkImports module

What does this MR do and why?

In 52d3a674 (MR: https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/3464) we deleted the actual regexes from Gitlab::Regex::BulkImports, leaving just the validation messages.

This change deletes the module as a follow-up clean up as it no longer has regexes.

#424432 (closed)

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Related to #424432 (closed)