[Task]: rm crontab.tpl and gen-cron.py

[eviljeff]

Description

Cron jobs running on addons-server are no longer defined by the output of gen-cron.py, they're separately defined in a deployment repo.

https://github.com/mozilla-it/webservices-infra/blob/main/amo/k8s/addons-server/values.yaml

Acceptance Criteria

  ### Milestones/checkpoints
  - [ ] obsolete files removed

Checks

• [X] If the issue is ready to work on, I have removed the "needs:info" label.
• [X] If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

┆Issue is synchronized with this Jira Task

[diox]

We should reference https://github.com/mozilla-it/webservices-infra/blob/main/amo/k8s/addons-server/values.yaml somewhere in the cron command code.