Remove Gallery feature related code and resources

[QilongTang]

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Purpose

Discussed with @Amoursol and UX content team, moving forward, Dynamo will more rely on guided tour and other mechanism to convey whats new content.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

Release Notes

Remove Gallery feature related code and resources

Reviewers

@DynamoDS/dynamo

FYIs

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[QilongTang]

@mjkkirschner If I remember correctly, we cant remove public properties in DynamoCore right? So adding Obsolete property, but I removed view layer commands and functions