[autograd.Function] Kill the extension feature flag (#92026)

This PR removes the autograd.Function extension feature flag. This was
previously used for development of the functorch <> autograd.Function
interaction.

It's been in master for long enough with the feature flag defaulting to
True, so it's time to remove it.

Test Plan:
- existing tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92026
Approved by: https://github.com/soulitzer

torch/csrc/autograd/function.h

@@ -60,15 +60,6 @@ class NodeGuard {
 // executed.
 TORCH_API std::shared_ptr<Node> get_current_node();
 
-// Global (not thread-local) feature flag for the new autograd.Function
-// extension. The extension consists of:
-// - splitting autograd.Function.forward into forward() and setup_context().
-// - adding a vmap staticmethod to autograd.Function
-// The feature flag is for preventing users from unknowningly stumbling upon
-// the feature and will be removed once we've ironed out the details.
-TORCH_API bool isAutogradFunctionExtensionEnabled();
-TORCH_API void setAutogradFunctionExtensionEnabled(bool enabled);
-
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 //                               Node
 //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~