feat: combine gprd main and cny pages

What

• Combine the pagerduty pages for stage when it's main and cny
• Delete the now unused RouteCase method.

Before	After

Why

In https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15765 we've started grouping alerts by the service this resulsted into group the alerts with that are firing for cny and main stages which is what we want. However we end up paging the on-call twice because we have 2 recivers as show in https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15964#problem.

We already show the stage if it's main or cny in the alert labels and in the alert description so it's already pretty clear wich stage is effected.

Reference: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15964