fix(ast-spec): remove duplicate union types from Expression

[MichaelDeBoey]

No description provided.

[typescript-eslint]

Thanks for the PR, @MichaelDeBoey!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[bradzacher]

Thanks for cleaning this up!