Related to https://github.com/grafana/oncall/pull/5116
Simplifies the db migration removing the `DeclaredIncident` model + FK
setup but keeping the other changes (adding `severity` field for
escalation policy, and "Declare incident" step, which is disabled). In
this way deployments for which the original migration was run, this
won't be applied and they will be in sync with the migration status
(eventually a manual step may be needed to remove the table and FK,
which won't be used for now).
4d9846eeb4