Remove this rule for several reasons:
* A single upvote usually isn't enough to matter, especially with the
new comment threshold.
* It felt weird that trying to vote on a comment could fail.
* Disabling the upvote button on your own comments feels weird.
* Most other sites allow you to upvote your own comments.
* You're allowed to upvote your own uploads, so it doesn't make sense
that you can't upvote your own comments.
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