Remove setup.py link after successful _undo_develop

This function reverts a previous setuptools 'develop' operation, but
doesn't actually clean up the artifacts under the build tree that it
used to determine that 'develop' was previously used. The result is that
after performing a build with --symlink-install, all subsequent builds
of that package without --symlink-install will run the commands to
"undo" the develop operation even if there is nothing to undo.

This should provide a small performance improvement under those specific
circumstances.

colcon_core/task/python/build.py

@@ -198,6 +198,8 @@ async def _undo_develop(self, pkg, args, env):
             ]
             completed = await run(
                 self.context, cmd, cwd=args.build_base, env=env)
+            if not completed.returncode:
+                os.remove(setup_py_build_space)
             return completed.returncode
 
     def _undo_install(self, pkg, args, setup_py_data, python_lib):