Drop retry_job_start_pipeline_after_commit feature flag

What does this MR do and why?

This drops the feature flag and makes the default behavior to start a pipeline in the after_commit hook. This fixes two issues:

1. Automatically retried builds were stuck in the pending state since multiple instances of Ci::Build were instantiated, but only one of the after_commit blocks was run. (#387775 (closed))

2. PipelineProcessWorker was previously scheduled in a transaction. Now it is done outside of a transaction. (#398229 (closed))

Relates to #403934 (closed)

How to set up and validate locally

See !116480 (merged).

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