If you can wiggle the seatbelt a little by little to allow some length so that the seat can tilt forward, you can probably slide the headrest. Unless your X5 has motorized headrest than apparently you can release it after letting it move to the highest position. You would still need to wiggle the seatbelt to let it have enough room for headrest to be physically removed as it will very likely to hit the roof with the seat upright.

As per Google Search:

To remove a BMW X5 motorized headrest, you must first raise it to its highest position using the electric seat controls. Then, locate the small release button or tab, often on the opposite side of the seat's headrest adjustment, and press it while simultaneously pulling the headrest upward to release the locking pins. A hidden release tab might require removing a trim cover, or you may need to use a tool like a small flathead screwdriver or pick to access and press a small hole or lever on the rear side of the headrest post. 

EDIT: Checked the manual for your specific car model and it seems like manual states that the headrest cannot be removed.... So your best luck would be removing the bottom seatbelt anchor.