Upcoming dismissal as supervisor no longer wants to supervise me - should I just withdraw?
Does your university have a mentoring system? I think you need to sit down with a mediator and the PI to try and work out a solution where you can continue either under the PI's guidance or someone else. If you do not have a mentor, then you should talk to another Professor who you think will be sympathetic and be able to guide you. Something is terribly wrong if you are being kicked out and it is a surprise. You need to set-up a meeting and negotiate a solution, a meeting with the department chair sounds a bit abrupt, unless they can help find a solution somehow.

I cannot give an answer as to whether you should withdraw. But if you were under the impression you can graduate this year, then withdrawing does not seem necessary. You will have wasted at least three years (presumably).