Alerts migration cleanup

[obostjancic]

In order to get alerts migration through, we added a lot of temporary code. This issue aims to remove all of that

Tasks

Preview Give feedback

1. (Frontend) make generic metrics the default alert dataset
2. chore(alerts): migration UI cleanup #61347
   Scope: Frontend +1
   
3. (Backend) remove validation?
4. chore(alerts): remove migration email templates #62461
   Scope: Backend +1
5. https://github.com/getsentry/getsentry/pull/12307
6. (Backend gs) remove migration email script
7. Remove related feature flags
8. Add custom metric alert type in alert list #60574
   +0
   
9. Drop table of migration
10. Remove redash dashboards/queries #85438
    +0