Remove UI for Opsgenie integration

Problem to Solve

We are replacing the minimal Opsgenie integration that exists today with our brand new HTTP endpoints. Users can easily set up an endpoint to route Opsgenie alerts to GitLab, if they want to surface them in the GitLab Alerts List!

Plan

See the detailed plan here.

Proposal

Remove the Opsgenie integration. I believe that includes the following:

Remove from Integrations drop-down

Remove config form

Remove state of Alerts list with Opsgenie integration is enabled

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