There are two standard types of hollow metal frame doors, knock down and welded. When dealing with a knockdown, the frame will have been installed previously in pieces. If there are screws on the face of the frame, it’s likely a knock down frame. First remove the door from the frame by removing the screws from the hinges. Remove the screws from the frame with a power drill and appropriate bit. Pull the jams off the wall by hand or pry them with a pry or crow bar if needed.
To remove a welded door frame, the frame may not simply come off with a pull or pry. The door may be anchored somewhere else that’s not visible, possibly near the floor. In between the jamb and the stud, run a reciprocating saw all the way up, sawing through any fasteners. Do this on both jams. Next, you’ll need to cut the jambs horizontally dividing it into sections. Usually a cut in the middle of each jamb is enough to remove the full jamb. Once the jambs are gone, try removing the head by hand. If it doesn’t come out try the pry bar, and lastly you can also cut it into sections if need be.
If you are dealing with a masonry wall and/or a jamb that’s been grouted solid, removal will be much more time consuming. You’ll need a powerful reciprocating saw or most likely a cut-off saw to cut the jambs into sections and cutting vertically at the juncture between the outside of the jamb and the wall.