change to setuptools in accordance with migration guide

Author: arne48

setup.py

@@ -1,8 +1,6 @@
 #!/usr/bin/env python
-try:
-    from setuptools import setup
-except ImportError:
-    from distutils.core import setup
+
+from setuptools import setup
 from catkin_pkg.python_setup import generate_distutils_setup
 
 d = generate_distutils_setup(