Can I be held liable if the non profit organization I no longer belong to doesn't remove my name as a signer from their checking account?

Being a signer on a checking account doesn't make you liable for anything except, in some cases, what you sign.  If all you mean is that you had authority to sign checks on behalf of the organization, that doesn't create any liability on your part.  Nor are you liable if someone forges your name, assuming you can demonstrate that the signature is not actually yours.  That being said, however, you have nothing to gain from having your name on the account (unless you, yourself, are planning some wrongdoing) and it could cause problems (although not ultimate liability) if people are misled into thinking that you continue to represent the organization, so it would be best if your name was removed.