fix to hide tooltip after removing subflow tab

[HiroyasuNishiyama]

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

Proposed changes

Closing subflow template tab while Hide flow tooltip is shown, leaves the tooltip shown.
This PR try to fix this issue by handling remove target event.

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 (-0.03%) to 67.307% when pulling 7959d18 on node-red-hitachi:fix-hide-flow-tooltip into b7bae18 on node-red:master.