Remove virtual keyword.

It was added in GitHub pull request #2895, but since fell
into disfavor compared to a more general way of negating
storage classes.

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Seems there is no PR open for actually removing `virtual`,
so I created one. This should be uncontroversial, because it
is a release blocker right now. Even if it doesn't make it into
2.066, `virtual` could still be added later.