Remove needless note about signals and classes.

As far as I know, any user-defined class must extend Object or a
subclass thereof, meaning it would be impossible to define a class that
couldn't use signals. This warning is therefore unnecessary.

getting_started/scripting/gdscript/gdscript_basics.rst

@@ -1424,11 +1424,6 @@ the :ref:`Object.connect() <class_Object_method_connect>` method::
         progress_bar.animate(old_value, new_value)
     ...
 
-.. note::
-
-    To use signals, your class has to extend the ``Object`` class or any
-    type extending it like ``Node``, ``KinematicBody``, ``Control``...
-
 In the ``Game`` node, we get both the ``Character`` and ``Lifebar`` nodes, then
 connect the character, that emits the signal, to the receiver, the ``Lifebar``
 node in this case.