Remove standalone seed job?

[damccorm]

I'm not sure what the historical reason for having two separate seed jobs is, but it looks like today the two are almost identical, so we can probably delete one of them.

$ diff .test-infra/jenkins/job_seed_standalone.groovy .test-infra/jenkins/job_00_seed.groovy

23c23
< job('beam_SeedJob_Standalone') {
---
> job('beam_SeedJob') {
77c77
< cron('0 */5 * * *')
---
> cron('H */6 * * *')
85c85
< triggerPhrase('Run Standalone Seed Job')
---
> triggerPhrase('Run Seed Job')
90c90
< context("Jenkins: Standalone Seed Job")
---
> context("Jenkins: Seed Job")
110c110
< python3.8 -m venv ve3 && source ./ve3/bin/activate &&
---
> python3.8 -m venv ve3 && source ve3/bin/activate &&

Imported from Jira BEAM-14226. Original Jira may contain additional context.
Reported by: ibzib.