aggregrate.h: Remove outer struct for enum ClassKind

[ibuclaw]

This was only used to give ClassKind a namespace in the absence of enum class being permitted.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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dub run digger -- build "master + dmd#11892"

[ibuclaw]

Azure CI didn't seem to get the pr/commit hook from GitHub, forcing a new build to trigger.