hw: allow bypassing "too old firmware" error when using hw wallets

The framework here is generic enough that it can be used for any hw plugin,
however atm only Trezor is implemented.

closes #5391

electrum/plugin.py

@@ -403,7 +403,7 @@ class DeviceMgr(ThreadJob):
 
     def unpair_xpub(self, xpub):
         with self.lock:
-            if not xpub in self.xpub_ids:
+            if xpub not in self.xpub_ids:
                 return
             _id = self.xpub_ids.pop(xpub)
             self._close_client(_id)