Remove 'Labels' and 'Milestones' page from under Merge Requests in the navigation

PLAN

Remove the 'Labels' page and "Milestones" page from underneath the 'Merge Requests' tab. Keep these pages under the 'Issues' tab.

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ORIGINAL DESCRIPTION

Summary

Selecting labels in the sub menu on the Merge Requests tab, opens up labels under the Issues tab

Steps to reproduce

1. Go to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests
2. Click on labels in the sub menu
3. Notice that the blue highlight bar is now on the Issues tab instead of the Merge Requests tab

Expected behavior

The blue highlight bar should be on Merge Requests

Actual behavior

The blue highlight bar is on Issues

Relevant logs and/or screenshots