Remove Duplicate Dashboard item from dashboards dropdown

What does this MR do?

When an out-of-the-box dashboard is being viewed in Operations > Metrics, the user has the option to duplicate this dashboard in two places:

• in the Dashboards Dropdown, by clicking the Duplicate dashboard item
• in the Actions menu, by clicking on Duplicate current dashboard

This MR removes the first option, i.e. the item in the Dashboards Dropdown.

Also, the docs are changed to reflect the removal here: https://docs.gitlab.com/ee/operations/metrics/dashboards/#duplicate-a-gitlab-defined-dashboard

Related issue: #223223 (closed)

Screenshots

Before	After

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