Don't show Author column on posts screen for single author sites

[johnbillion]

The 'Author' column on post listing screens is a bit pointless for single-author sites.

To simplify this screen, I think it would make sense to remove this column for sites which only have one user, and possibly also remove it for sites with more than one user but still only one author (ie. !is_multi_author()).

I'm not sure how best to handle the column's visibility and user's column visibility preferences at the point when a site goes from being a single-author site to a multi-author site.