re-enable install

The Big Split is done enough that installation works again.

setup.py

@@ -160,23 +160,6 @@ def run(self):
     'unsymlink': unsymlink,
 }
 
-### Temporarily disable install while it's broken during the big split
-from textwrap import dedent
-from distutils.command.install import install
-
-class DisabledInstall(install):
-    def run(self):
-        msg = dedent("""
-        While we are in the midst of The Big Split,
-        IPython cannot be installed from master.
-        You can use `pip install -e .` for an editable install,
-        which still works.
-        """)
-        print(msg, file=sys.stderr)
-        raise SystemExit(1)
-
-setup_args['cmdclass']['install'] = DisabledInstall
-
 
 #---------------------------------------------------------------------------
 # Handle scripts, dependencies, and setuptools specific things