Fix clear pinned paths of debug sidebar menu

[HiroyasuNishiyama]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Selecting Clear pinned paths of debug sidebar item menu for global message causes editor runtime error.

debug-utils.js:468 Uncaught TypeError: activeMenuMessage.clearPinned is not a function
    at Object.onselect (debug-utils.js:468:43)
    at f (red.min.js?v=3.0.0-beta.4:18:47386)
    at HTMLAnchorElement.<anonymous> (red.min.js?v=3.0.0-beta.4:18:46029)
    at HTMLAnchorElement.dispatch (vendor.js?v=3.0.0-beta.4:2:43090)
    at v.handle (vendor.js?v=3.0.0-beta.4:2:41074)

This PR try to fix the issue.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.02%) to 68.311% when pulling 5ae5669 on node-red-hitachi:fix-debug-path-menu into 8cb212d on node-red:master.