How to delete document in content version?

In the SFDC Objects documentation you referenced, at the top of the page is this line:

create(), describeLayout(), describeSObjects(), query(), 
retrieve(), search(), update(), upsert()

No delete - hence a DML delete on this object is not permitted.

This makes sense because in the user interface, you can't delete a version either, only add a new one

All ContentVersion versions are hung under (as a child) a ContentDocument. The ContentDocument, if deleted, deletes all child ContentVersions

From the SFDC Object reference documentation on ContentDocument:

When you delete a document, all versions of that document are deleted, including ratings, comments, and tags.