Remove the bell from the calculator

Don't ping the terminal bell on every key-press. This might be okay on some
systems, on others (we're looking at you Windows) it's a wee bit
distracting.

See #955.

examples/calculator.py

@@ -93,8 +93,6 @@ class CalculatorApp(App):
         button_id = event.button.id
         assert button_id is not None
 
-        self.bell()  # Terminal bell
-
         def do_math() -> None:
             """Does the math: LEFT OPERATOR RIGHT"""
             try: