Remove help messages for legacy setup.py commands

The help messages added in 45cdbb1
(Added help messages for removed setup.py commands) have been displayed
for almost five years by now, so they have fulfilled their purpose.

setup.py

@@ -4,67 +4,9 @@
 # NOTE: The configuration for the package, including the name, version, and
 # other information are set in the pyproject.toml file.
 
-import sys
+from setuptools import setup
 
-# First provide helpful messages if contributors try and run legacy commands
-# for tests or docs.
-
-TEST_HELP = """
-Note: running tests is no longer done using 'python setup.py test'. Instead
-you will need to run:
-
-    tox -e test
-
-If you don't already have tox installed, you can install it with:
-
-    pip install tox
-
-If you only want to run part of the test suite, you can also use pytest
-directly with::
-
-    pip install -e .[test]
-    pytest
-
-For more information, see:
-
-  https://docs.astropy.org/en/latest/development/testguide.html#running-tests
-"""
-
-if "test" in sys.argv:
-    print(TEST_HELP)
-    sys.exit(1)
-
-DOCS_HELP = """
-Note: building the documentation is no longer done using
-'python setup.py build_docs'. Instead you will need to run:
-
-    tox -e build_docs
-
-If you don't already have tox installed, you can install it with:
-
-    pip install tox
-
-You can also build the documentation with Sphinx directly using::
-
-    pip install -e .[docs]
-    cd docs
-    make html
-
-For more information, see:
-
-  https://docs.astropy.org/en/latest/install.html#builddocs
-"""
-
-if "build_docs" in sys.argv or "build_sphinx" in sys.argv:
-    print(DOCS_HELP)
-    sys.exit(1)
-
-
-# Only import these if the above checks are okay
-# to avoid masking the real problem with import error.
-from setuptools import setup  # noqa: E402
-
-from extension_helpers import get_extensions  # noqa: E402
+from extension_helpers import get_extensions
 
 ext_modules = get_extensions()