port: Microsoft.Bot.Schema Cleanup (Part 2) (#5605)

[github-actions[bot]]

The changes in Microsoft.Bot.Schema Cleanup (Part 2) (#5605) may need to be ported to maintain parity with microsoft/botbuilder-dotnet.

Part 2. Fixes #5737

This piece contains the cleanup of Microsoft.Bot.Schema -- small things such as removing unnecessary usage of this. keyword, simplifying default value expressions, etc.

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Per Monica, breaking up PR#5538 into 2 pieces, due to the size of PR needed to bring Schema up to 100% code coverage

Merge these cleanup pieces after we merge in the unit test coverage

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• Unit tests here in Part 1 PR
• Code Cleanup in .Schema in Part 2 PR

Please review and, if necessary, port the changes.