Remove payload_key_for method

Remove `PrometheusAlertEvent.payload_key_for` and
`SelfManagedPrometheusAlertEvent.payload_key_for` methods
in favor of `Gitlab::Alerting::Alert#gitlab_fingerprint`

app/models/prometheus_alert_event.rb

@@ -34,10 +34,4 @@ class PrometheusAlertEvent < ApplicationRecord
   def self.status_value_for(name)
     state_machines[:status].states[name].value
   end
-
-  def self.payload_key_for(gitlab_alert_id, started_at)
-    plain = [gitlab_alert_id, started_at].join('/')
-
-    Digest::SHA1.hexdigest(plain)
-  end
 end

app/models/self_managed_prometheus_alert_event.rb

@@ -15,10 +15,4 @@ class SelfManagedPrometheusAlertEvent < ApplicationRecord
       yield event if block_given?
     end
   end
-
-  def self.payload_key_for(started_at, alert_name, query_expression)
-    plain = [started_at, alert_name, query_expression].join('/')
-
-    Digest::SHA1.hexdigest(plain)
-  end
 end