Use Bootstrap Flex utility to remove .d-flex-center

What does this MR do?

Use Bootstrap Flex utility to remove unused CSS .d-flex-center (gitlab-org/gitlab-ce!26134).

What are the relevant issue numbers?

Follows gitlab-org/gitlab-ce!26134

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated via this MR
• [n/a] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n/a] Conforms to the database guides
• [n/a] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• [n/a] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• [n/a] Security reports checked/validated by reviewer