Drop abort_user_pipelines_on_block feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes :abort_user_pipelines_on_block toggle to conclude https://gitlab.com/gitlab-org/gitlab/-/issues/324045.

This also removes CancelUserPipelinesService because it isn't used anymore.

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done