Do not export PrometheusAlerts

It requires dependency to Environments
and it does not make sense without knowledge
of deployments.

spec/lib/gitlab/import_export/all_models.yml

@@ -355,7 +355,9 @@ priorities:
 - label
 prometheus_metrics:
 - project
-- prometheus_alert
+- prometheus_alerts
+prometheus_alerts:
+- project
 timelogs:
 - issue
 - merge_request

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -596,6 +596,12 @@ PrometheusMetric:
 - group
 - common
 - identifier
+PrometheusAlert:
+- threshold
+- operator
+- environment_id
+- project_id
+- prometheus_metric_id
 Badge:
 - id
 - link_url