Show alerts settings only for manual configuration

What does this MR do?

Show alerts settings only for manual configuration

We should hide alerts settings from GitLab-managed Prometheus instances and show them only in manual mode.

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/9258#note_132612251

Before	After

What are the relevant issue numbers?

Found when testing 11.8.0-rc1 QA Issue.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

/cc @joshlambert @psimyn