withdrow experimental widget node generation code in order to reorganize node generation model

[HiroyasuNishiyama]

Types of changes

What types of changes does your code introduce?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

In order to discuss APIs for supporting installable widget node, we will temporallily withdrow experimental support of widget node generation code from nodegen.

Checklist

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• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/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 increased (+1.5%) to 76.389% when pulling daddb3a on node-red-hitachi:withdrow-widget-template into 3537fa3 on node-red:master.