Nuke and deprecate sexual

BUR #26432 is pending approval.

nuke sexual
deprecate sexual

Ambiguous single word tag and on a post that was taken down so cannot be edited.

I removed it from the banned post. But it's still worth deprecating.

Not sure if it’s worth adding more clutter to the list of deprecated tags for a tag that has been used only three times in the history of Danbooru.