Small clean up for media list table

[ocean90]

• $t_time was never used.
• Media tags aren't supported by core.

The tags support is like the custom taxonomies on the write page, don't see a reason to take it out.

We don't yet support custom taxonomy columns on the write page (though we should).

We shouldn't remove the column handler. We can however remove commented-out line adding it to $post_columns.

That said, I'm more interested in the other aspect of the patch, the cleanup of timestamps. Yes, it is possible for media to be unpublished, but probably nto through the API. But the rest of it looks interesting.