[functorch] Get rid of defunct functorch/setup.py (#87235)

We initially left it there for BC concerns.
- It has been more than a month since then,
- I have migrated folks who used the previous install command (pip
install ...pytorch.git@subdir=functorch) off of it

so it's time to get rid of it

Test Plan:
- code reading
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87235
Approved by: https://github.com/Chillee

functorch/setup.py

@@ -1,57 +0,0 @@
-# Copyright (c) Facebook, Inc. and its affiliates.
-# All rights reserved.
-#
-# This source code is licensed under the BSD-style license found in the
-# LICENSE file in the root directory of this source tree.
-# This is a dummy setup.py that does not do anything
-
-import os
-import subprocess
-from setuptools import setup
-import warnings
-import torch
-
-cwd = os.path.dirname(os.path.abspath(__file__))
-
-try:
-    sha = subprocess.check_output(['git', 'rev-parse', 'HEAD'], cwd=cwd).decode('ascii').strip()
-except Exception:
-    sha = 'Unknown'
-package_name = 'functorch'
-
-requirements = [
-    # This represents a nightly version of PyTorch.
-    # It can be installed as a binary or from source.
-    "torch>=1.13.0.dev",
-]
-
-extras = {}
-extras["aot"] = ["networkx", ]
-
-
-if __name__ == '__main__':
-    try:
-        setup(
-            # Metadata
-            name=package_name,
-            version=torch.__version__,
-            author='PyTorch Core Team',
-            url="https://github.com/pytorch/functorch",
-            description='JAX-like composable function transforms for PyTorch',
-            license='BSD',
-
-            # Package info
-            packages=[],
-            install_requires=requirements,
-            extras_require=extras,
-        )
-    except Exception as e:
-        print(e, file=sys.stderr)
-        sys.exit(1)
-
-    warnings.warn(
-        'Installing PyTorch from source or from a nightly binary already '
-        'installs functorch (as of 9/14/2022), so there is no need to cd '
-        'into functorch and run `python setup.py {install, develop}` anymore. '
-        'We will soon remove this method of installing functorch.',
-        DeprecationWarning)