Remove `vulnerability_occurrence_pipelines` table and code

What does this MR do and why?

This table is excessively large because it is a many-to-many relationship between vulnerability findings and ci pipelines. We've removed all usages of this table from the application (#450797 (closed)), so it can now be dropped and all code associated with it can be deleted.

There is an enqueued cells background migration named BackfillVulnerabilityOccurrencePipelinesProjectId operating on the table. To avoid errors, we delete all the background migration jobs before dropping the table. The migration enqueuing the job has been changed to a no-op so that the BBM code can be safely deleted.

Relates to: #422386 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.