If you just want to remove the fabric and leave it bare, just do that. Cut away the parts that have let loose, or remove all of it. The fiber board and remaining adhesive will be what's left where the fabric is removed. It won't look pretty. If recovering or replacing it, it needs to come out, which isn't difficult at all in an extended cab (barn doors, with no B pillar in the way). If you want to make it somewhat presentable, you can remove it, strip it of all the fabric and adhesive, then paint it. I've seen some textured coatings that blend right in and look nice, if noticeable at all (some manufacturers actually do this instead of using a fabric covering).