Remove the fallback to on-the-fly calculation of latest pipeline with security reports

Why are we doing this work

By #271408 (closed) we've started using a pre-set value for retrieving the latest pipeline with security reports instead of calculating the value by using the ci_pipelines table which was using expensive SQL queries. Since we had to run a migration to populate the values, we've implemented a fallback mechanism to use the previous logic to return the value if there isn't one set already. We should remove that fallback along with the only_successful flag as the flag is wrong anyway.

Implementation plan

• backend Remove the only_successful flag from the latest_pipeline_with_security_reports method
• backend Remove the fallback method latest_not_ingested_security_pipeline from the latest_pipeline_with_security_reports method.