Just understood that I can’t delete an “Article Date” column I’ve added to a document library.
After a brief googling I’ve found an article by John F. Holiday (SharePoint Server MVP), where he sais, that following columns can’t be deleted after you’ve added them to an object:
Contact, Contact E-Mail Address, Contact Name, Contact Picture,
Scheduling Start Date, Scheduling End Date,
Byline, Image Caption,
Page Content, Page Icon, Page Image,
Summary Links, Summary Links 2.
That's it, you can't delete it :(
Anyway, he gives a sample code which could help with deletion. Didn't tried yet, just've hidden the column.