Remove "Attached To" column from the attachments table

[scribu]

Before 3.5, an attachment's parent post meant "this is the post this file belongs to; it will automatically show up in the gallery". In 3.5, if it's even set, it means "this is the post this file was originally uploaded to".

After just ​seeing a user trying to insert an image into a post using the "Attached To" column (around 2:30), I'm thinking maybe we should hide that column.