use solver=None by default

src/pymgrid/algos/mpc/mpc.py

@@ -376,7 +376,7 @@ def _create_problem(self, eta, battery_capacity, fuel_cost, cost_battery_cycle,
     def _get_solver(self, failure=False):
         if not failure:
             logger.info("Using default solver." if self._passed_solver is None else f"Using {self._passed_solver} solver.")
-            return self._passed_solver or cp.CLARABEL
+            return self._passed_solver
 
         # failure