Remove PagerDuty receiver for migration to incident.io

Alex Hanselkarequested to merge
amh-pagerduty-to-incidentio into master

This MR removes the PagerDuty receiver for alertmanager as part of our migration to incident.io On-Call. This is a part of the change issue gitlab-com/gl-infra/production#20792 (closed).

Edited by Alex Hanselka

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