Remove unnecessary CI jobs

[victorlin]

Description of proposed changes

GitHub Actions runs are free, but that doesn't mean all of them are useful. Remove the ones that aren't serving much purpose.

Checklist

• [x] Checks pass
• [x] ~If making user-facing changes, add a message in CHANGELOG.md summarizing the changes in this PR~
• [x] ~(to be done by a Nextstrain team member) Create preview PRs on downstream repositories.~