Drop legacy submit_message & groups columns from jobs

[tgucks]

What type of PR is this?

Data migration / cleanup

What this PR does / why we need it

Final step of the submit_message / groups migration from the jobs table to the dedicated job_metadata table (see the prior PRs here: first, second, and third).

This PR:

• Adds migration 040_drop_jobs_legacy_metadata_columns.sql, which drops submit_message and groups from the jobs table. From this PR forward, submit_message and groups live exclusively in job_metadata.
• Removes the JobMetadataMigrationPhase config field from both the scheduler and scheduleringester, along with the legacy / dualWrite / cutover switching logic that read it.
• Deletes the now-dead migration helpers (job_metadata_migration.go) and the phase-specific tests.

Special notes for your reviewer

• Do not merge until the prior PRs (second, and third) are merged. JobMetadataMigrationPhase needs to be set to cutover prior to dropping the columns. Once pt-3 is rolled out, this PR is safe.
• This PR removes the config flags JobMetadataMigrationPhase in scheduler & scheduleringester so they must be removed from any custom configs.