Remove panel via vuex in dashboard

What does this MR do?

Background Moving forward our dashboard will be saved back to the backend, so we need the frontend to keep an update vuex store if the user makes changes.

Issue Currently the use can remove a panel from the dashboard (when rearrangePanelsAvailable is true) however, the change is done by removing the object directly from the object in the dashboard and skipping a commit to the store.

This fix ensures this is done using a Vuex store action instead.

⚠ You can enable rearrangePanelsAvailable in the dashboard to see the draggable/removable panels in action.

Screenshots

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

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

Related to #27835 (closed)