Remove logic for opsgenie integration

What does this MR do?

Removes logic for the deprecated opsgenie integration.

• Related issue: #292991 (closed)
• UI removal issue: #273657 (closed)
• Frontend removal MR: !50525 (merged)

Out of scope: database record removal. I plan to remove the corresponding AlertsService records as a part of #255501 (closed), as the entire service will be obsolete at that point.

I'm opting not to include a changelog here as there are no user-facing changes in this MR.

Verification plan

• In a project with no saved alert integrations, see that opsgenie is not listed as an integration type option (Settings > Operations > Alerts)
• Test that an HTTP integration can be created & saved
• Test that alerts list is viewable

In a removal, the best we can do is test that other stuff doesn't break, so I figure it makes sense to simply test that the replacement functionality for the opsgenie integration (which is HTTP Integrations) works.

Save integration & send test alert	View alert list	Delete integration

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team