Remove managed-cluster alert code and deprecation feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

The GitLab-managed Kuberenetes cluster alerts feature was removed by flipping the managed_alerts_deprecation feature flag to default-off in 14.1. This MR:

• Removes the managed-cluster alerts disabled code.
• Removes the :managed_alerts_deprecation feature flag.

Background:

• In GitLab 14.0 and earlier there was a GitLab-managed-cluster feature, whereby GitLab would install and manage applications on a connected Kubernetes cluster.
• This feature was deprecated in 14.0 and removed in 14.1.
• This change meant the alerts feature for Metrics dashboards is no longer useable, so the UI was disabled behind a feature flag at the same time.
  • Epic here: &5877

To test the changes:

• Set up a kubernetes cluster, install Prometheus on to the cluster. Ensure that Prometheus is available from the GDK.
• Connect Prometheus to the GitLab instance. Settings -> Integrations -> Prometheus.
• Deploy at least one environment to the cluster.
• View the metrics dashboard.

Issue: #331863 (closed)

Screenshots (strongly suggested)

None - no visual change.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [n/a] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [n/a] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [n/a] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [n/a] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

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