I hope this message finds you well, present circumstances excluded. This is an interesting case for a number of reasons. The most obvious issue that comes to mind is that you essentially have two different contracts at the outset (contract for the equipment and an insurance contract for the equipment). You have been removed from the equipment contract and no one can refute that simply fact.
As I understand it, even though you no longer have an equity interest in the equipment whatsoever, they are attempting to still enforce your original liability on the insurance.
Since you have no equity interests in the equipment, it is counter intuitive for the opposing party to claim you cannot cancel your insurance agreement over said equipment. Moreover, they should not be able to use your signature on the insurance agreement as a binding act of law relative to the equipment.
If there is a penalty provision in the insurance agreement for early withdrawal, you may be contractually bound to pay that penalty, in whole or in part (may be able to prorate). However, you are not legally bound to stay in the contract and insurance property to which you have no legal obligation. As such, your partner in assuming payments for the equipment must necessarily assume the insurance coverage as well.
In short, barring a penalty provision in the insurance contract to which you may owe a fine for early withdrawal, you have no legal responsibility in this situation based on the information that you have shared with me.
Let me know if you have any additional questions or comments.
Best wishes moving forward!