I can add existing site columns to a document library, but I can't delete them. For example I added a Description site column, but I can't remove it.
When you add a page content field it will created as sealed column and by default sharepoint doesn't allow to delete sealed columns. So to delete it via pwoershell.
$web = Get-SPWeb “https://sharepoint.comm/” //this will get site collection reference $list = $web.Lists["List Name "] //get the list / library where Column is added $field = $list.Fields["Columns name "] // get the field /column which you want delete $field.Allowdeletion = $true //no allow for deletion $field.Sealed = $false //Now break the seal $field.Delete() //deleted $list.Update() //Update the list.