revert change from r32955.

This added a poll function to the Panel base class but this means that all panels register as having a poll() function which is called a lot and does nothing, raising an exception and removing it because of how python implements hasattr().

Found when checking why exceptions were continuously raised on redraw.

Martin, can you check on netrender?

release/scripts/modules/bpy_types.py

@@ -723,12 +723,6 @@ class _GenericUI:
 class Panel(StructRNA, _GenericUI, metaclass=RNAMeta):
     __slots__ = ()
 
-    @classmethod
-    def poll(cls, context):
-        # super class might not have a poll function, better be careful
-        cls = super()
-        return not hasattr(cls, "poll") or cls.poll(context)
-
 
 class Header(StructRNA, _GenericUI, metaclass=RNAMeta):
     __slots__ = ()